It used to clear just issue number and patchset, but that's not
all state that associates a given branch with codereview site.

R=andybons@chromium.org
BUG=609295

Review-Url: https://codereview.chromium.org/2032433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300683 0039d316-1c4b-4281-b951-d872f2087c98