Ticket patchset revision delete functionality

[paulsputer]

Adding functionality to delete patchset revision as discussed: https://groups.google.com/forum/#!topic/gitblit/JKh92WSQ_d4

Key aspects:

• Update git revision reference using update-ref command
• Update ticket journal
  • Should maintain append-only concept of journal
• This interface would prevent deleting merged history, i.e can only delete rebased non-referenced items