status: advertise --abort instead of 'update -C .' to abort a merge status has a part where it shows the conflict information and how to continue or abort. Couple of release ago, we got merge --abort and we should advertise that instead of 'hg update -C .' which is kind of ugly. I know we need to unify the logic here. Differential Revision: https://phab.mercurial-scm.org/D4168