I was initially worried about the cost of always snapshotting the
working copy, so that's why e.g. `jj diff -r <some hash>` doesn't do
it. However, there's been a few caused by missing snapshotting, and
there are still a few (I just noticed it in `jj undo` while writing
this patch). Let's always do the snapshotting and if the user really
doesn't want it, they can pass `--no-commit-working-copy` (which we
should probably rename to `--no-snapshot-working-copy` or maybe just
`--no-snapshot`). That should reduce bugs and make the CLI more
predictable.
Two test cases were affected becasue `jj merge` also didn't snapshot
the working copy.
Before this patch, e.g. `jj co --no-commit-working-copy` would error
out, but now it will succeed (without touching the working copy,
leaving the working copy stale). That may be confusing, but it should
be easy to recover from (e.g. by `jj undo`). We can consider adding a
check for it later if it seems too confusing (it's probably rarely
something the user wanted).
We had tests for `jj move` and `jj squash`, but not for `jj
unsquash`. I'm about to add a `--keep` option, so let's add tests for
the current behavior first.
