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

git_common.py

Index: git_common.py
diff --git a/git_common.py b/git_common.py
index 946a39d5d2655968bbcad3e9df5397046643018f..a18f59f16474d19e5ed52085dedc70af42f2be8e 100644
--- a/git_common.py
+++ b/git_common.py
@@ -399,7 +399,7 @@ def get_or_create_merge_base(branch, parent=None):
   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)