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.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown