py3: fix error string with bytestr() on repr()d value

Differential Revision: https://phab.mercurial-scm.org/D3131

mercurial/mdiff.py

@@ -78,7 +78,8 b' class diffopts(object):'
             self.context = int(self.context)
         except ValueError:
             raise error.Abort(_('diff context lines count must be '
-                               'an integer, not %r') % self.context)
+                                'an integer, not %r') %
+                              pycompat.bytestr(self.context))
 
     def copy(self, **kwargs):
         opts = dict((k, getattr(self, k)) for k in self.defaults)