remote-hg: force remote bookmark push selectively

If we update the 'old' node, we might be updating the remote bookmark
even when our 'new' node is not related at all to what the remote has,
effectively forcing an update.

Let's do that only when forced push is configured.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/remote-helpers/git-remote-hg

@@ -919,7 +919,8 @@ def do_export(parser):
         # update remote bookmarks
         remote_bmarks = peer.listkeys('bookmarks')
         for ref, bmark, old, new in p_bmarks:
-            old = remote_bmarks.get(bmark, '')
+            if force_push:
+                old = remote_bmarks.get(bmark, '')
             if not peer.pushkey('bookmarks', bmark, old, new):
                 print "error %s" % ref
     else: