stringutil: have buildrepr delegate to pprint for unknown types
This stabilizes some revset reprs across Python 2 and 3.
Differential Revision: https://phab.mercurial-scm.org/D4244
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