Merged be.diff-subscribe
Highlights:
* changes to `be diff`
* exits with an error if required revision control is not possible.
Previously it printed a message, but exitted with status 0.
* removed options --new, --removed, --modified, --all
* added options --uuids, --subscribe
* New method diff.Diff.full_report() allows fast generation of
similar report_tree()s via diff.DiffTree.masked.
* New method diff.subscriptions_from_string() for consistent
subscription string parsing.
* clean up be-handle-mail.Message.subscriber_emails() with
diff.Diff.report_tree(subscriptions)
* hardcoded 'DIR' replaced with diff.BUGDIR_ID
* assorted cleanups and bugfixes