Make git auto-svn idempotent.

By directly using git-config rather than git-svn-init, this ensures that
crazy values don't get set. It could be "safer" (e.g. check to see if any
other git-svn configuration already exists and prompt before overwriting it),
but I think that simplicity is better here.

R=pgervais@chromium.org
BUG=425838

Review URL: https://codereview.chromium.org/675713002

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292640 4ff67af0-8c30-449e-8e8b-ad334ec8d88c