Fixed libbe.command.diff + ugly BugDir.duplicate_bugdir implementation

Fixed libbe.command.diff + ugly BugDir.duplicate_bugdir implementation

duplicate_bugdir() works, but for the vcs backends, it could require
shelling out for _every_ file read.  This could, and probably will, be
horribly slow.  Still it works ;).

I'm not sure what a better implementation would be.  The old
implementation checked out the entire earlier state into a temporary
directory
  pros: single shell out, simple upgrade implementation
  cons: wouldn't work well for HTTP backens

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.
I'm stuck on how to handle upgrades though...

Also removed trailing whitespace from all python files.