This is in-place of --remotes. With repos like src.git that have alternate roots
this causes git-map to be faster (since it's not trying to display
origin/master).

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/212183003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@259646 0039d316-1c4b-4281-b951-d872f2087c98