py3: fix a bytes vs str issue in remotefilelog extension

Differential Revision: https://phab.mercurial-scm.org/D7598

hgext/remotefilelog/fileserverclient.py

@@ -663,5 +663,5 b' class fileserverclient(object):'
         self.ui.log(
             b'remotefilelog',
             b'excess remotefilelog fetching:\n%s\n',
-            b''.join(traceback.format_stack()),
+            b''.join(pycompat.sysbytes(traceback.format_stack())),
         )