stringutil: have buildrepr delegate to pprint for unknown types

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

--- a/mercurial/utils/stringutil.py	Fri Aug 10 01:41:31 2018 -0400
+++ b/mercurial/utils/stringutil.py	Fri Aug 10 01:42:01 2018 -0400
@@ -114,7 +114,7 @@
     elif callable(r):
         return r()
     else:
-        return pycompat.byterepr(r)
+        return pprint(r)
 
 def binary(s):
     """return true if a string is binary data"""