Refactor git_common config and die

R=iannucci@chromium.org
BUG=376099

Review-Url: https://codereview.chromium.org/2075603002
diff --git a/git_upstream_diff.py b/git_upstream_diff.py
index 3e38c78..c9fdce6 100755
--- a/git_upstream_diff.py
+++ b/git_upstream_diff.py
@@ -11,7 +11,7 @@
 import git_common as git
 
 def main(args):
-  default_args = git.config_list('depot-tools.upstream-diff.default-args')
+  default_args = git.get_config_list('depot-tools.upstream-diff.default-args')
   args = default_args + args
 
   parser = argparse.ArgumentParser()