py3: use '%d' for integers instead of '%s'

I think this is the last of these occurences because I have got the test passing
on python3 in a later patch.

Differential Revision: https://phab.mercurial-scm.org/D5862

hgext/convert/monotone.py

@@ -154,7 +154,7 b' class monotone_source(common.converter_s'
         retval = []
         while True:
             commandnbr, stream, length, output = self.mtnstdioreadpacket()
-            self.ui.debug('mtn: read packet %s:%s:%s\n' %
+            self.ui.debug('mtn: read packet %s:%s:%d\n' %
                 (commandnbr, stream, length))
 
             if stream == 'l':