git-cl: Fix 'git cl issue --reverse'

While ripping SVN support out of git-cl, the Changelist() constructor
changed to require that the branch name start with "refs/heads/",
but the CMDissue --reverse code was never updated to feed it the
fully-qualified refname.

R=thestig

Bug: 791176
Change-Id: Ia7fee0f77fec080d91f34ad43bfd9f7c6a4bf64b
Reviewed-on: https://chromium-review.googlesource.com/806239
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -4533,7 +4533,7 @@ def CMDissue(parser, args):
 
   if options.reverse:
     branches = RunGit(['for-each-ref', 'refs/heads',
-                       '--format=%(refname:short)']).splitlines()
+                       '--format=%(refname)']).splitlines()
     # Reverse issue lookup.
     issue_branch_map = {}
     for branch in branches: