Merged "be subscribe" and be-handle-mail subscription support.

Also assorted other changes and fixes in the be.subscribe branch.
Highlights:
  * Much more powerful libbe.diff with subclassable report generators.
  * "be diff" compares working copy with last commit by default.
  * comment reference text shown in "be comment" EDITOR footer
  * .revision_id() for all VCSs
  * meaningful comment comparison and stricter bug comparison
  * stricter .sync_with_disk interpretation.  See BugDir.__doc__.
  * Comment.From and .time_string -> .author and .date, for better
    conformance with settings_object.setting_name_to_attr_name().