stringutil: teach pprint about sets

This is the old (Python 2) way of printing sets. I actually prefer the
Python 3 version of the repr, but this will result in less test churn
in the short term.

Differential Revision: https://phab.mercurial-scm.org/D4243

mercurial/utils/stringutil.py

@@ -59,6 +59,9 b' def pprint(o, bprefix=False):'
             '%s: %s' % (pprint(k, bprefix=bprefix),
                         pprint(v, bprefix=bprefix))
             for k, v in sorted(o.items())))
+    elif isinstance(o, set):
+        return 'set([%s])' % (b', '.join(
+            pprint(k, bprefix=bprefix) for k in sorted(o)))
     elif isinstance(o, tuple):
         return '(%s)' % (b', '.join(pprint(a, bprefix=bprefix) for a in o))
     else: