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).
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.
