It seems simpler to do the snapshotting after merging any concurrent
operations instead of snapshotting on top of one of the operations,
especially since the attempt to snapshot may end up noticing that the
working copy is stale.
More importantly, snapshotting before resolving operations resulted in
a crash if the working copy was modified. That happened because we
held a lock on the operation heads (`locked_op_heads`) while we tried
to record the operation committing the working copy. I noticed this
only after adding the test.
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.
It's useful for testing to be able to specify some operation that's
not the latest one.
I didn't update the changelog because this feature is mostly for
testing.
This involved copying `UnresolvedHeadRepo::resolve()` into the CLI
crate (and modifying it a bit to print number of rebased commit),
which is unfortunate.
