To reduce server load, only fetch the ref we need.

This is a redo of: http://codereview.chromium.org/2856038

First upload was the original CL 2856038. Subsequent uploads
are my changes.

TEST=ran presubmit tests

Review URL: http://codereview.chromium.org/2842060

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@53072 0039d316-1c4b-4281-b951-d872f2087c98