discovery: use stable sort order in --new-branch warning

A Python set seems to do this already when you iterate over it, but
this should be considered an implementation detail and not something
we can rely on.

This also gets rid of an unnecessary string conversion -- the set
already contains strings.

mercurial/discovery.py

@@ -280,7 +280,7 b' def prepush(repo, remote, force, revs, n'
             remotemap = remote.branchmap()
             newbranches = branches - set(remotemap)
             if newbranches and not newbranch: # new branch requires --new-branch
-                branchnames = ', '.join("%s" % b for b in newbranches)
+                branchnames = ', '.join(sorted(newbranches))
                 repo.ui.warn(_("abort: push creates "
                                "new remote branches: %s!\n")
                              % branchnames)