discovery: re-wrap expression to avoid a black bug

Differential Revision: https://phab.mercurial-scm.org/D6969

mercurial/discovery.py

@@ -394,8 +394,10 b' def checkheads(pushop):'
             if len(newhs) > 1:
                 dhs = list(newhs)
                 if errormsg is None:
-                    errormsg = (_("push creates new branch '%s' "
-                                  "with multiple heads") % (branch))
+                    errormsg = (
+                        _("push creates new branch '%s' with multiple heads") %
+                        branch
+                    )
                     hint = _("merge or"
                              " see 'hg help push' for details about"
                              " pushing new heads")