Logic for highlighting "origin/master" (etc) just looked for "origin"
as a prefix. Add the slash since "origin-foo" is a boring old valid
topic branch name.

R=iannucci@chromium.org, iannucci@google.com

Review URL: https://codereview.chromium.org/1832973002 .

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299486 0039d316-1c4b-4281-b951-d872f2087c98