Previously a `[depot-tools "rebase-update"]` section would show up in the
.git/config on every `git rebase-update` cycle.

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/228353003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@263423 0039d316-1c4b-4281-b951-d872f2087c98