git-cl-patch: fix --is-ancestor flag

TBR=tandrii@chromium.org

Bug: 723787
Change-Id: Ie5017747f2070116774cd12ba0dd2ea531b2d0aa
Reviewed-on: https://chromium-review.googlesource.com/529547
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 7ccf8e0..1c61897 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2796,10 +2796,10 @@
     RunGit(['fetch', fetch_info['url'], fetch_info['ref']])
 
     clean, _ = RunGitWithCode(
-        ['merge-base', '--ancestor', 'HEAD', 'origin/master'])
+        ['merge-base', '--is-ancestor', 'HEAD', 'origin/master'])
     if clean != 0:
       clean, _ = RunGitWithCode(
-          ['merge-base', '--ancestor', 'HEAD', 'FETCH_HEAD'])
+          ['merge-base', '--is-ancestor', 'HEAD', 'FETCH_HEAD'])
     if clean != 0:
       confirm_or_exit(
           'It looks like you\'re on a branch with some local commits.\n'