Otherwise, the old merge-base value can confuse the hell out of `git cl upload`.

BUG=none

Review URL: https://codereview.chromium.org/252683003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@266648 0039d316-1c4b-4281-b951-d872f2087c98