When rebasing a changeset X and that changeset becomes empty, we should move
the bookmark on X to rebase destination.
This is a regression caused by the scmutil.cleanupnodes refactoring for
rebase.
The `adjustdest` function calculates the destination of bookmark movement.
It was back-ported from https://phab.mercurial-scm.org/D21. It might be
slightly more powerful than the minimal requirement to solve this issue.
For example, it's impossible for a merge changeset to become empty while any
of its ancestors does not become empty, but the code could handle that case.
Since the code is reasonably short and clean, and helps the upcoming D21
series, I'd like to check-in `adjustdest` now.
Thanks Martin von Zweigbergk for spotting corner cases (-k and descendant
with bookmarks) in this area!
