templater: don't blow up when trying to build an abort message

__name__ is unicode, but we need bytes. For now, we'll make the
(mostly-safe) assumption that template filter names will be ascii.

Differential Revision: https://phab.mercurial-scm.org/D1137

mercurial/templater.py

@@ -427,7 +427,7 b' def runfilter(context, mapping, data):'
         sym = findsymbolicname(arg)
         if sym:
             msg = (_("template filter '%s' is not compatible with keyword '%s'")
-                   % (filt.__name__, sym))
+                   % (filt.__name__.encode('ascii'), sym))
         else:
             msg = _("incompatible use of template filter '%s'") % filt.__name__
         raise error.Abort(msg)