incoming: use bytes for an error message

Caught by pytype.

Differential Revision: https://phab.mercurial-scm.org/D10461

mercurial/hg.py

@@ -1272,7 +1272,7 b' def _incoming('
     srcs = urlutil.get_pull_paths(repo, ui, [source], opts.get(b'branch'))
     srcs = list(srcs)
     if len(srcs) != 1:
-        msg = _('for now, incoming supports only a single source, %d provided')
+        msg = _(b'for now, incoming supports only a single source, %d provided')
         msg %= len(srcs)
         raise error.Abort(msg)
     source, branches = srcs[0]