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