hg grpush --strict option

Created originally on Bitbucket by marmoute (Pierre-Yves David)

I make a very seldom use of bare "hg push" in my repository. I usually have a lot of unfinished business in my repo that I'm not willing to push. I would like to introduce a --strict option to hg grpush that will push revision referenced in .hgguestrepo only.

I'll provide a patch if people agree