Handle (ignore) binary file diffs

If git diff detects a binary file with differences, it might add
some (currently) unparseable text to the diff. Ignore it to avoid an
error.

Change-Id: Ib4f615e2f2bd8d633657949c8732556b4a0575c6

gertty/gitrepo.py

@@ -454,6 +454,8 @@ class Repo(object):
                     continue
                 if line.startswith("index"):
                     continue
+                if line.startswith("Binary files"):
+                    continue
                 if not last_line:
                     raise Exception("Unhandled line: %s" % line)
             f.finalize()