Allow refs/branch-heads

This is necessary for uploading v8 changes.

Bug: None
Change-Id: I2847c4bcddacf1985c0c230b892716942ae2b031
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5332374
Auto-Submit: Eric Leese <leese@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Eric Leese <leese@chromium.org>

git_cl.py

@@ -1311,7 +1311,7 @@ class Changelist(object):
 
         self.branchref = branchref
         if self.branchref:
-            assert branchref.startswith('refs/heads/')
+            assert branchref.startswith(('refs/heads/', 'refs/branch-heads/'))
             self.branch = scm.GIT.ShortBranchName(self.branchref)
         else:
             self.branch = None