subrepo: introduce hgsubrepo._getctx()

This code is already used in a couple of places, and will need to be used in
others where wdir() support is needed.  Trying to get the wdir() revision stored
in self._substate[1] when creating the object in subrepo.subrepo() resulted in
breaking various status and diff tests.  This is a far less invasive change.

mercurial/subrepo.py

@@ -662,6 +662,15 b' class hgsubrepo(abstractsubrepo):'
         finally:
             lock.release()
 
+    def _getctx(self):
+        '''fetch the context for this subrepo revision, possibly a workingctx
+        '''
+        if self._ctx.rev() is None:
+            return self._repo[None]  # workingctx if parent is workingctx
+        else:
+            rev = self._state[1]
+            return self._repo[rev]
+
     @annotatesubrepoerror
     def _initrepo(self, parentrepo, source, create):
         self._repo._subparent = parentrepo