> I think a good solution would run along the lines of the currently
> commented out code in duplicate_bugdir(), where a
>   VersionedStorage.changed_since(revision)
> call would give you a list of changed files.  diff could work off of
> that directly, without the need to generate a whole duplicate bugdir.

This is definately the way to go.  Rough approach for the VCS family:

1) Parse `bzr diff` or such to get a list of new,changed,moved,removed
   paths.
2) Convert those paths to ids.
3) Return a list of ids to duplicate_bugdir().
4) Provide Storage.parent(id, revision), so duplicate_bugdir() could
   figure out what type of id we were dealing with (bugdir, bug,
   comment, other?), and construct the appropriate difference tree.

There could be a DupBugDir class which stored that diff tree and a
link to the current bugdir, which would make diffs much easier (work
already done, just copy the diff tree), and provide faster access to
unchanged files (just use the current version).