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).
If the source commit becomes empty as a result of
`move/squash/unsquash`, we abandon it. However, perhaps we shouldn't
do that if the source commit is a working-copy commit because
working-copy commits are often work-in-progress commits.
The background for this change is that @arxanas had just started a new
change and had set a description on it, and then decided to make some
changes in the working copy that should be in the parent
commit. Running `jj squash` then abandoned the working-copy commit,
resuling in the description getting lost.
It can be confusing that some commits (typically the working copy)
don't have a description. Let's show a placeholder text in such cases.
I chose the format to match the "(no email configured)" message we
already have.
