remote-hg: safer bookmark pushing

It is possible that the remote has changed the bookmarks, so let's fetch
them before we make any assumptions, just the way mercurial does.

Probably doesn't make a difference, but better be safe than sorry.

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

contrib/remote-helpers/git-remote-hg

@@ -782,6 +782,8 @@ def do_export(parser):
             continue
 
         if peer:
+            rb = peer.listkeys('bookmarks')
+            old = rb.get(bmark, '')
             if not peer.pushkey('bookmarks', bmark, old, new):
                 print "error %s" % ref
                 continue