cmdutil: provide a way to report how to continue
checkafterresolved allows Mercurial to suggest what command to
use next. If users try to continue the wrong command, there
wasn't a good way for the command to suggest what to do next.
Split checkmdutil into howtocontinue and checkafterresolved.
Introduce wrongtooltocontinue which handles raising an Abort with
the hint from howtocontinue.