This command uses an external merge tool to resolve conflicts
simple enough that they can be resolved with a 3-way merge.
This commit provides a very basic version of `jj resolve` that
is hardcoded to use vimdiff.
This also slightly changes the errors of the Diff Editor, so that
both the diff editor and `jj resolve can share an error type.
Let's acknowledge everyone's contributions by replacing "Google LLC"
in the copyright header by "The Jujutsu Authors". If I understand
correctly, it won't have any legal effect, but maybe it still helps
reduce concerns from contributors (though I haven't heard any
concerns).
Google employees can read about Google's policy at
go/releasing/contributions#copyright.
I feel the original -------/+++++++ pair is slightly confusing because
each half can be a separator by itself. I don't know what character other
than '-'/'+' is preferred, but let's pick '%' (for "mod") per @martinvonz
suggestion.
In 8ae9540f2c, I made `jj move/squash/unsquash` not abandon the
working copy if it became empty because that would lose any
description associated with it. It turned out that the new behavior
was also confusing because it made it unclear if the working-copy
commit was actually abandoned. Let's roll back that change and instead
ask the user for a combined description when both the source and
destination commits have non-empty descriptions. Not discarding a
non-empty description seems like a good improvement regardless of the
behavior related to working-copy commits. It's also how `hg fold`
behaves (though hg doesn't allow the description to be empty).
