Fix silly bug in presubmit_support implementation of AppendCC.

The executor needs to accumnulate more_cc, not overwrite them.

Bug: 786386
Change-Id: Iadfe836ae9623f1b66d2db8209d347b5e61ccae8
Reviewed-on: https://chromium-review.googlesource.com/784553Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>

presubmit_support.py

@@ -1318,7 +1318,7 @@ class PresubmitExecuter(object):
         logging.debug('Running %s in %s', function_name, presubmit_path)
         result = eval(function_name + '(*__args)', context)
         logging.debug('Running %s done.', function_name)
-        self.more_cc = output_api.more_cc
+        self.more_cc.extend(output_api.more_cc)
       finally:
         map(os.remove, input_api._named_temporary_files)
       if not (isinstance(result, types.TupleType) or