mw-sort-wmf-branches: sort wmf branchs numerically

Very basic sorter for the origin/wmf/X.YYwmfZ wmf branches being used
for MediaWiki repositories.  Will let us find out the latest branch
available by tailing the last item from the sorted list.

Example usage:

 $ git branch -r --list '*/wmf/*wmf*'|mw-sort-wmf-branches.py|tail -n3
   gerrit/wmf/1.24wmf3
   gerrit/wmf/1.24wmf4
   gerrit/wmf/1.24wmf5
 $

1.24wmf5 being the last branch 8-)

Bug: 62509
Change-Id: Ic20779bbc94e93f2b0ac60a9cc6412f1737833d2