ui: convert stack traces to sysbytes before logging

They're coming back as unicodes, so sysbytes seems like the best we can do.

This is like D2171 and D2172, but those fail on Python 2.7.5
(seriously!), so this is my version of the same change. I actually
wrote this before reviewing those, then discarded it, then came back
to it after finding out 2.7.5 is a silly place.

Differential Revision: https://phab.mercurial-scm.org/D2209

mercurial/ui.py

@@ -1620,10 +1620,12 b' class ui(object):'
         else:
             curframe = inspect.currentframe()
             calframe = inspect.getouterframes(curframe, 2)
-            self.write_err('%s at: %s:%s (%s)\n'
-                           % ((msg,) + calframe[stacklevel][1:4]))
-            self.log('develwarn', '%s at: %s:%s (%s)\n',
-                     msg, *calframe[stacklevel][1:4])
+            fname, lineno, fmsg = calframe[stacklevel][1:4]
+            fname, fmsg = pycompat.sysbytes(fname), pycompat.sysbytes(fmsg)
+            self.write_err('%s at: %s:%d (%s)\n'
+                           % (msg, fname, lineno, fmsg))
+            self.log('develwarn', '%s at: %s:%d (%s)\n',
+                     msg, fname, lineno, fmsg)
             curframe = calframe = None  # avoid cycles
 
     def deprecwarn(self, msg, version, stacklevel=2):