Make marked merge base invalid when the upstream changes.

git_mark_merge_base.py

Index: git_mark_merge_base.py
diff --git a/git_mark_merge_base.py b/git_mark_merge_base.py
index cefb6df9cf2e0023c1a1038ad1f25e92f27dded5..673e2b433220c518959f7cf836612696243e3eab 100755
--- a/git_mark_merge_base.py
+++ b/git_mark_merge_base.py
@@ -17,7 +17,7 @@ import sys
 from subprocess2 import CalledProcessError
 
 from git_common import remove_merge_base, manual_merge_base, current_branch
-from git_common import get_or_create_merge_base, hash_one
+from git_common import get_or_create_merge_base, hash_one, upstream
 
 
 def main(argv):
@@ -51,7 +51,7 @@ def main(argv):
       )
       return 1
 
-    manual_merge_base(cur, opts.merge_base)
+    manual_merge_base(cur, opts.merge_base, upstream(cur))
 
   ret = 0
   actual = get_or_create_merge_base(cur)