buildman: Fix incorrect arguemnt in GetUpstream()

This causes an error when trying to build a local branch which has a local
branch as its upstream.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

tools/patman/gitutil.py

@@ -129,7 +129,7 @@ def GetUpstream(git_dir, branch):
         return upstream, msg
 
     if remote == '.':
-        return merge
+        return merge, None
     elif remote and merge:
         leaf = merge.split('/')[-1]
         return '%s/%s' % (remote, leaf), None