This causes the 'git config' to only add or update the branch-heads fetch
entry, and not replace any other existing entry (e.g. refs/heads/*).

Review URL: https://chromiumcodereview.appspot.com/12634013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@189326 0039d316-1c4b-4281-b951-d872f2087c98