minirst: explicitly decode substitutions

Caught by Augie and sys.setdefaultencoding("undefined")

mercurial/minirst.py

@@ -56,7 +56,7 b' def replace(text, substs):'
     # on strings in local encoding causes invalid byte sequences.
     utext = text.decode(encoding.encoding)
     for f, t in substs:
-        utext = utext.replace(f, t)
+        utext = utext.replace(f.decode("ascii"), t.decode("ascii"))
     return utext.encode(encoding.encoding)
 
 _blockre = re.compile(r"\n(?:\s*\n)+")