readme: improved frontmatter, part 2

Summary: What was going to just be some minor touch-ups to the existing content
ended in another rework of the frontmatter, this time primarily the sales pitch
and basic feature explanation.

The motivation here is simple: you should not just encounter a three-word noun
that is a hyperlink to pages with 1,000 words actually explaining the three-word
noun itself is. It's jarring!

Instead, the frontmatter is longer, expanding on each major selling point and
similarity to other tools. It actually *describes* the important, distinct
design decisions that tell you what the tool is and does, rather than just link
you around a bunch.

For example, one immediate thing is that calling jj a "DVCS" is actually kind
of odd when it later becomes apparent that you can have multiple data model and
commit backends; Google for example uses it in a more centralized manner than
others would via Piper/CitC. Calling it a "DVCS" is a bit strange in this sense
when *really* what we mean is that the Git data model allows independent copies
of the repo.

Overall I think this is *much* better for people who are just going to see the
README and may or may not bounce off immediately.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: I9f0f78e56157ef434ec239710e00f3bd

README.md

@@ -23,21 +23,103 @@
 
 ## Introduction
 
-Jujutsu is a
+Jujutsu is a powerful [version control system](https://en.wikipedia.org/wiki/Version_control)
-[Git-compatible](https://martinvonz.github.io/jj/latest/git-compatibility)
+for software projects. You use it to get a copy of your code, track changes
-[DVCS](https://en.wikipedia.org/wiki/Distributed_version_control). It combines
+to the code, and finally publish those changes for others to see and use.
-features from Git (data model,
+It is designed from the ground up to be easy to use—whether you're new or
-[speed](https://github.com/martinvonz/jj/discussions/49)), Mercurial (anonymous
+experienced, working on brand new projects alone, or large scale software
-branching, simple CLI [free from "the
+projects with large histories and teams.
-index"](https://martinvonz.github.io/jj/latest/git-comparison#the-index),
+
-[revsets](https://martinvonz.github.io/jj/latest/revsets), powerful
+Jujutsu is unlike most other systems, because internally it abstracts the user
-history-rewriting), and Pijul/Darcs ([first-class
+interface and version control algorithms from the *storage systems* used to
-conflicts](https://martinvonz.github.io/jj/latest/conflicts)), with features not
+serve your content. This allows it to serve as a VCS with many possible physical
-found in most of them
+backends, that may have their own data or networking models—like [Mercurial] or
-([working-copy-as-a-commit](https://martinvonz.github.io/jj/latest/working-copy),
+[Breezy], or hybrid systems like Google's cloud-based design, [Piper/CitC].
-[undo functionality](https://martinvonz.github.io/jj/latest/operation-log),
+
-automatic rebase, [safe replication via `rsync`, Dropbox, or distributed file
+[Mercurial]: https://www.mercurial-scm.org/
-system](https://martinvonz.github.io/jj/latest/technical/concurrency)).
+[Breezy]: https://www.breezy-vcs.org/
+[Piper/CitC]: https://youtu.be/W71BTkUbdqE?t=645
+
+Today, we use Git repositories as a storage layer to serve and track content,
+making it **compatible with many of your favorite Git-based tools, right now!**
+All core developers use Jujutsu to develop Jujutsu, right here on GitHub. But it
+should hopefully work with your favorite Git forges, too.
+
+We combine many distinct design choices and concepts from other version control
+systems into a single tool. Some of those sources of inspiration include:
+
+- **Git**: We make an effort to [be fast][perf]—with a snappy UX, efficient
+  algorithms, correct data structures, and good-old-fashioned attention to
+  detail. The default storage backend uses Git repositories for "physical
+  storage", for wide interoperability and ease of onboarding.
+
+- **Mercurial & Sapling**: There are many Mercurial-inspired features, such as
+  the [revset] language to select commits. There is [no explicit index][no-index]
+  or staging area. Branches are "anonymous" like Mercurial, so you don't need
+  to make up a name for each small change. Primitives for rewriting history are
+  powerful and simple. Formatting output is done with a robust template language
+  that can be configured by the user.
+
+- **Pijul & Darcs**: Jujutsu keeps track of conflicts as [first-class objects][conflicts]
+  in its model; they are first-class in the same way commits are, while
+  alternatives like Git simply think of conflicts as textual diffs. While not
+  as rigorous as based systems like Darcs and Pijul (which are based on
+  a formalized theory of patches, as opposed to snapshots), the effect is
+  that many forms of conflict resolution can be performed and propagated
+  automatically.
+
+[perf]: https://github.com/martinvonz/jj/discussions/49
+[revset]: https://martinvonz.github.io/jj/latest/revsets/
+[no-index]: https://martinvonz.github.io/jj/latest/git-comparison/#the-index
+[conflicts]: https://martinvonz.github.io/jj/latest/conflicts/
+
+And it adds several innovative, useful features of its own:
+
+- **Working-copy-as-a-commit**: Changes to files are [recorded automatically][wcc]
+  as normal commits, and amended on every subsequent change. This "snapshot"
+  design simplifies the user-facing data model (commits are the only visible
+  object), simplifies internal algorithms, and completely subsumes features like
+  Git's stashes or the index/staging-area.
+
+- **Operation log & undo**: Jujutsu records every operation that is performed on the
+  repository, from commits, to pulls, to pushes. This makes debugging problems like
+  "what just happened?" or "how did I end up here?" easier, *especially* when
+  you're helping your coworker answer those questions about their repository!
+  And because everything is recorded, you can undo that mistake you just made
+  with ease. Version control has finally entered [the 1960s][undo-history]!
+
+- **Automatic rebase and conflict resolution**: When you modify a commit, every
+  descendent is automatically rebased on top of the freshly-modified one. This
+  makes "patch-based" workflows a breeze. If you resolve a conflict in a commit,
+  the _resolution_ of that conflict is also propagated through descendants as
+  well. In effect, this is a completely transparent version of `git rebase
+  --update-refs` combined with `git rerere`, supported by design.
+
+  > [!WARNING]
+  > The following features are available for use, but experimental; they may
+  > have bugs, backwards incompatible storage changes, and user-interface
+  > changes!
+
+- **Safe, concurrent replication**: Have you ever wanted to store your version
+  controlled repositories inside a Dropbox folder? Or continuously backup
+  repositories to S3? No? Well, now you can!
+
+  The fundamental problem with using filesystems like Dropbox and backup tools
+  like `rsync` on your typical Git/Mercurial repositories is that that they rely
+  on *local filesystem operations* being atomic, serialized, and non-concurrent
+  with respect to other reads and writes—which is _not_ true when operating on
+  distributed file systems, or when operations like concurrent file copies (for
+  backup) happen while lock files are being held.
+
+  Jujutsu is instead designed to be [safe under concurrent scenarios][conc-safety];
+  simply using rsync or Dropbox and then using that resulting repository
+  should never result in a repository in a *corrupt state*. The worst that
+  _should_ happen is that it will expose conflicts between the local and remote
+  state, leaving you to resolve them.
+
+[wcc]: https://martinvonz.github.io/jj/latest/working-copy/
+[undo-history]: https://en.wikipedia.org/wiki/Undo#History
+[conc-safety]: https://martinvonz.github.io/jj/latest/technical/concurrency/
 
 The command-line tool is called `jj` for now because it's easy to type and easy
 to replace (rare in English). The project is called "Jujutsu" because it matches
@@ -49,14 +131,11 @@ questions, or want to talk about future plans, please join us on Discord
 or start a [GitHub Discussion](https://github.com/martinvonz/jj/discussions); the
 developers monitor both channels.
 
-> [!IMPORTANT]
-> Jujutsu is an **experimental version control system**. While Git compatibility
-> is stable, and most developers use it daily for all their needs, there may
-> still be work-in-progress features, suboptimal UX, and workflow gaps that make
-> it unusable for your particular use.
-
 ### News and Updates 📣
 
+- **Oct 2023**: Version 0.10.0 is released! Now includes a bundled merge and
+  diff editor for all platforms, "immutable revsets" to avoid accidentally
+  `edit`-ing the wrong revisions, and lots of polish.
 - **Jan 2023**: Martin gave a presentation about Google's plans for Jujutsu at
   Git Merge 2022! See the
   [slides](https://docs.google.com/presentation/d/1F8j9_UOOSGUN9MvHxPZX_L4bQ9NMcYOp1isn17kTC_M/view)
@@ -64,6 +143,12 @@ developers monitor both channels.
 
 ## Getting started
 
+> [!IMPORTANT]
+> Jujutsu is an **experimental version control system**. While Git compatibility
+> is stable, and most developers use it daily for all their needs, there may
+> still be work-in-progress features, suboptimal UX, and workflow gaps that make
+> it unusable for your particular use.
+
 Follow the [installation
 instructions](https://martinvonz.github.io/jj/latest/install-and-setup) to
 obtain and configure `jj`.
@@ -89,17 +174,22 @@ the header of the website when you scroll to the top of any page.
 
 ### Compatible with Git
 
-Jujutsu has two
+Jujutsu is designed so that the underlying data and storage model is abstract.
-[backends](https://martinvonz.github.io/jj/latest/glossary#backend). One of them
+Today, it features two [backends]—one of them uses a Git repository for storage,
-is a Git backend (the other is a native one [^native-backend]). This lets you
+while the other is a native storage backend[^native-backend].
-use Jujutsu as an alternative interface to Git. The commits you create will look
+
-like regular Git commits. You can always switch back to Git. The Git support
+[backends]: https://martinvonz.github.io/jj/latest/glossary#backend
-uses the [libgit2](https://libgit2.org/) C library.
 
 [^native-backend]: At this time, there's practically no reason to use the native
 backend. The backend exists mainly to make sure that it's possible to eventually
 add functionality that cannot easily be added to the Git backend.
 
+The Git backend is fully featured and maintained, and allows you to use Jujutsu
+as an alternative interface to Git. The commits you create will look like
+regular Git commits. You can always switch back to Git. The Git support uses the
+[libgit2](https://libgit2.org/) C library.
+
+
 <img src="demos/git_compat.png" />
 
 You can even have a ["co-located" local
@@ -179,12 +269,14 @@ commit to any other commit using `jj move`.
 
 The tool is fairly feature-complete, but some important features like (the
 equivalent of) `git blame` are not yet supported. There
-are also several performance bugs. It's also likely that workflows and setups
+are also several performance bugs. It's likely that workflows and setups
-different from what the core developers use are not well supported.
+different from what the core developers use are not well supported, e.g. there
+is no native support for email-based workflows.
 
-I (Martin von Zweigbergk) have almost exclusively used `jj` to develop the
+Today, all core developers use `jj` to work on `jj`. I (Martin von Zweigbergk)
-project itself since early January 2021. I haven't had to re-clone from source
+have almost exclusively used `jj` to develop the project itself since early
-(I don't think I've even had to restore from backup).
+January 2021. I haven't had to re-clone from source (I don't think I've even had
+to restore from backup).
 
 There *will* be changes to workflows and backward-incompatible changes to the
 on-disk formats before version 1.0.0. Even the binary's name may change (i.e.