This will clean up index.lock and refs/heads/master.lock-style locks when
invoked as part of the update_scripts step on the bots.
R=dpranke@chromium.org, estaab@chromium.org, pgervais@chromium.org
BUG=421769
Review URL: https://codereview.chromium.org/1862943002
git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299737 0039d316-1c4b-4281-b951-d872f2087c98