py3: force hgweb.server error log to internally write unicode

Otherwise, there's a lot of py2/py3 divergence in the LFS tests because of the
"HG error" lines picking up a b'' prefix.  wsgicgi.py uses procutil.stderr, so I
assume the input was meant to be bytes.

mercurial/hgweb/server.py

@@ -54,7 +54,7 b' class _error_logger(object):'
         self.writelines(str.split('\n'))
     def writelines(self, seq):
         for msg in seq:
-            self.handler.log_error("HG error:  %s", msg)
+            self.handler.log_error(r"HG error:  %s", encoding.strfromlocal(msg))
 
 class _httprequesthandler(httpservermod.basehttprequesthandler):