templatekw: fix dict construction in _showlist to not mix bytes and strs

What we had was fine on Python 2, but was slightly wrong on Python
3. This works on both.

Differential Revision: https://phab.mercurial-scm.org/D2554

mercurial/templatekw.py

@@ -205,7 +205,9 b' def _showlist(name, values, templ, mappi'
             yield separator.join(values)
         else:
             for v in values:
-                yield dict(v, **strmapping)
+                r = dict(v)
+                r.update(mapping)
+                yield r
         return
     startname = 'start_' + plural
     if startname in templ: