diff -r 417fa23017f9 -r 2105bdd9462a mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Thu Dec 14 22:30:03 2017 -0800
+++ b/mercurial/debugcommands.py	Wed Dec 13 11:19:24 2017 -0800
@@ -744,8 +744,7 @@
 def debugdiscovery(ui, repo, remoteurl="default", **opts):
     """runs the changeset discovery protocol in isolation"""
     opts = pycompat.byteskwargs(opts)
-    remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl),
-                                      opts.get('branch'))
+    remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl))
     remote = hg.peer(repo, opts, remoteurl)
     ui.status(_('comparing with %s\n') % util.hidepassword(remoteurl))