hgweb: pass a sysstr to low-level _start_response method

This fixes a regression in Python 3 support introduced in 7de7bd407251
on the stable branch. We're so early in do_hgweb that I don't see any
especially better choices than this.

Differential Revision: https://phab.mercurial-scm.org/D3726

mercurial/hgweb/server.py

@@ -125,8 +125,9 b' class _httprequesthandler(httpservermod.'
         # Ensure the slicing of path below is valid
         if (path != self.server.prefix
             and not path.startswith(self.server.prefix + b'/')):
-            self._start_response(common.statusmessage(404), [])
-            self._write("Not Found")
+            self._start_response(pycompat.strurl(common.statusmessage(404)),
+                                 [])
+            self._write(b"Not Found")
             self._done()
             return