Add OAuth2 support for end users (i.e. 3-legged flow with the browser).

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 41ec428ead6cfa3c329367ef119973e021c06f3a..3c49ff2a30feb671fa82f1a0ae6dbdd434e0d76b 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -3193,6 +3193,8 @@ def main(argv):
   dispatcher = subcommand.CommandDispatcher(__name__)
   try:
     return dispatcher.execute(OptionParser(), argv)
+  except auth.AuthenticationError as e:
+    DieWithError(str(e))
   except urllib2.HTTPError, e:
     if e.code != 500:
       raise