dispatch: use flag cmdalias.shell for detecting shell aliases

mercurial/dispatch.py

@@ -295,7 +295,7 b' class cmdalias(object):'
             ui.debug("alias '%s' shadows command '%s'\n" %
                      (self.name, self.cmdname))
 
-        if self.definition.startswith('!'):
+        if hasattr(self, 'shell'):
             return self.fn(ui, *args, **opts)
         else:
             try: