revert: show warning when reverting subrepos that do not support revert

The warning is similar to the warning that was shown before hgsubrepo revert
support was added, with the exception that now the subrepo type is shown.

For example, when trying to revert a git subrepo located in "include/mygitsub",
the warning message would be:

include/mygitsub: reverting git subrepos is unsupported

mercurial/subrepo.py

@@ -369,6 +369,8 b' class abstractsubrepo(object):'
         return []
 
     def revert(self, ui, substate, *pats, **opts):
+        ui.warn('%s: reverting %s subrepos is unsupported\n' \
+            % (substate[0], substate[2]))
         return []
 
 class hgsubrepo(abstractsubrepo):