py3: convert traceback representation to bytes when logging

Python's APIs should return a system string. We want to use bytes
for logging. So convert the traceback to bytes before sending
it to the logger.

Differential Revision: https://phab.mercurial-scm.org/D2176

mercurial/dispatch.py

@@ -992,6 +992,7 b' def handlecommandexception(ui):'
     this function returns False, ignored otherwise.
     """
     warning = _exceptionwarning(ui)
-    ui.log("commandexception", "%s\n%s\n", warning, traceback.format_exc())
+    ui.log("commandexception", "%s\n%s\n", warning,
+           pycompat.sysbytes(traceback.format_exc()))
     ui.warn(warning)
     return False  # re-raise the exception