Previously this relied on being able to track branches which didn't have a
remote refspec. Now we add a fake origin for the git-svn test repo, and use
--prefix so that git has a remote to track and the refs are correctly
namespaced.

Should also be backwards compatible with older versions of git.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/61893006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@233649 0039d316-1c4b-4281-b951-d872f2087c98