trychange.py: create patch refs in refs/patches/
trychange.py: create patch refs in refs/patches/

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@263439 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools