git-mfc: Allow merging merge commits

In order to merge merge commits (such as vendor imports), we need to
tell git cherry-pick which of the two branches referenced in the commit
is the mainline.  In our case, it is always the first.

Approved by:	markj

tools/tools/git/git-mfc

@@ -358,7 +358,7 @@ def pending(repo, upstream, author=None, baking=False):
 
 def cherry_pick(commit, edit=False):
     """Cherry-pick a single commit with -x.  Abort on failure."""
-    cmd = ['git', 'cherry-pick', '-x']
+    cmd = ['git', 'cherry-pick', '-x', '-m1']
     if edit:
         cmd.append('-e')
     cmd.append(commit.hexsha)