WKR gets stuck when there are many merges
https://bugs.webkit.org/show_bug.cgi?id=114632

Reviewed by Andreas Kling.

Move the last revision forward when it exists in a branch.
Otherwise, we'll be stuck looping over revisions made to merge patches.

* Scripts/webkitpy/tool/commands/newcommitbot.py:
(NewCommitBot.next_work_item):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148461 268f45cc-cd09-0410-ab3c-d52691b4dbfc