subrepo: implement 'unshare' for Mercurial subrepos
I think there's a slight hole here in that a subrepo could be shared, removed
from .hgsub, and then it's not part of context.substate (so not iterated over).
But the push command has the same hole IIRC, and I think removing a subrepo is
an edge case.
The import hack is a copy/paste of subrepo.subrepo().