This add --force to ignore the flag file.
This also passes --base_ref so we can specify which ref we're basing off of so that it does not incorrectly assert that it is basing off origin/master

BUG=339171

Review URL: https://codereview.chromium.org/171763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@252464 0039d316-1c4b-4281-b951-d872f2087c98