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: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@292640 0039d316-1c4b-4281-b951-d872f2087c98