This is necesary to force an existing repo working directory to check
out a new version of the repo tool which has been signed with a new
key.

R=cmp@chromium.org

Review URL: https://codereview.chromium.org/22909004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@217347 0039d316-1c4b-4281-b951-d872f2087c98