Add checks to GitSanityChecks (upstream branch or current branch can be None)

git_common.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Monkeypatch IMapIterator so that Ctrl-C can kill everything properly.
 # Derived from https://gist.github.com/aljungberg/626518
 import multiprocessing.pool
 from multiprocessing.pool import IMapIterator
 def wrapper(func):
   def wrap(self, timeout=None):
@@ skipping 381 matching lines @@
 
 
 def get_or_create_merge_base(branch, parent=None):
   """Finds the configured merge base for branch.
 
   If parent is supplied, it's used instead of calling upstream(branch).
   """
   base = branch_config(branch, 'base')
   base_upstream = branch_config(branch, 'base-upstream')
   parent = parent or upstream(branch)
-  if not parent:
+  if parent is None or branch is None:
     return None
   actual_merge_base = run('merge-base', parent, branch)
 
   if base_upstream != parent:
     base = None
     base_upstream = None
 
   def is_ancestor(a, b):
     return run_with_retcode('merge-base', '--is-ancestor', a, b) == 0
 
@@ skipping 349 matching lines @@
         hash=branch_hash, upstream=upstream_branch, ahead=ahead, behind=behind)
 
   # Set None for upstreams which are not branches (e.g empty upstream, remotes
   # and deleted upstream branches).
   missing_upstreams = {}
   for info in info_map.values():
     if info.upstream not in info_map and info.upstream not in missing_upstreams:
       missing_upstreams[info.upstream] = None
 
   return dict(info_map.items() + missing_upstreams.items())