subrepo: make subrepo.subrepo(<not a subrepo path>) fail

Until now, when calling subrepo.subrepo with a path at which there is no
subrepo, a "nullstate" tuple would be returned. However, this is not very
useful (the tuple can't really be used for creating a subrepo), so we'd just as
soon have the function just fail, and leave it up to the caller to decide what
to do.

The motivation for doing this now is to simplify the solution for (issue3056).

mercurial/subrepo.py

@@ -268,7 +268,7 b' def subrepo(ctx, path):'
     hg = h
 
     scmutil.pathauditor(ctx._repo.root)(path)
-    state = ctx.substate.get(path, nullstate)
+    state = ctx.substate[path]
     if state[2] not in types:
         raise util.Abort(_('unknown subrepo type %s') % state[2])
     return types[state[2]](ctx, path, state[:2])