subrepo: make a message translatable

This message has been overlooked by check-code, because it starts with
non-alphabet character ('%').

This is also a part of preparation for making "missing _() in ui
message" detection of check-code more exact.

mercurial/subrepo.py

@@ -585,7 +585,7 b' class abstractsubrepo(object):'
         return 1
 
     def revert(self, substate, *pats, **opts):
-        self.ui.warn('%s: reverting %s subrepos is unsupported\n' \
+        self.ui.warn(_('%s: reverting %s subrepos is unsupported\n') \
             % (substate[0], substate[2]))
         return []