py3: add a missing b'' in tests/bruterebase.py

Differential Revision: https://phab.mercurial-scm.org/D2503

tests/bruterebase.py

@@ -65,7 +65,7 def debugbruterebase(ui, repo, source, d
                             desc += getdesc(prev)
                     descs.append(desc)
                 descs.sort()
-                summary = ' '.join(descs)
+                summary = b' '.join(descs)
             ui.popbuffer()
             repo.vfs.tryunlink(b'rebasestate')