httprepo: remove is-comparison with string literal

An identity check between a variable and a string literal was added to
the pushkey implementation in 6bd9778ae749. While CPython will
normally intern strings and thus make the test safe, value identity is
what should be used here.

mercurial/httprepo.py

@@ -68,7 +68,7 b' class httprepository(wireproto.wirerepos'
         raise util.Abort(_('operation not supported over http'))
 
     def _callstream(self, cmd, **args):
-        if cmd is 'pushkey':
+        if cmd == 'pushkey':
             args['data'] = ''
         data = args.pop('data', None)
         headers = args.pop('headers', {})