bisect: correct message about aborting an in-progress bisect (
issue6527)
By using a custom cmdhint message here we're avoiding automatic hint generation
in _statecheck.hint(), which would suggest 'hg bisect --continue' and 'hg
bisect --abort' (neither of which is a valid option).
This patch is only fixing the message about in-progress bisect, it doesn't
modify the unfinished state checking logic.