gitk: More bug fixes and cleanups

gitk: More bug fixes and cleanups

* Add/remove fake commits (for local changes) when updating the view
  even if nothing else has changed.
* Get rid of unused getdbg variable.
* Get rid of vseeds and uat.
* Fix bug where removerow would throw a "no such element in array" error.
* Clear out cached highlights when line numbers change.
* Make dodiffindex remove the fake commit rows if they currently exist
  but there are now no local changes.

Signed-off-by: Paul Mackerras <paulus@samba.org>