rebase: don't rebase obsolete commits with no successor
This patch avoids unnecessary conflicts to resolve during rebase for the users
of changeset evolution.
This patch modifies rebase to skip obsolete commits with no successor.
It introduces a new rebase state 'revpruned' for these revisions that are
being skipped and a new message to inform the user of what is happening.
This feature is gated behind the config flag experimental.rebaseskipobsolete
When an obsolete commit is skipped, the output is:
note: not rebasing 7:360bbaa7d3ce "O", it has no successor