In Git-mode, the previous version of code created a ref in
refs/heads/refs/patches/...  instead of refs/patches/....
This commit fixes the problem.

R=maruel@chromium.org, agable@chromium.org
BUG=362264

Review URL: https://codereview.chromium.org/233913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@263439 0039d316-1c4b-4281-b951-d872f2087c98