This should insert the message according to Gerrit's own commit-msg
hook implementation.

BUG=579183

Review URL: https://codereview.chromium.org/1758943002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299093 0039d316-1c4b-4281-b951-d872f2087c98