templater: show repr of string we're rejecting

I feel like this should make it a little easier to hunt down problems.

Differential Revision: https://phab.mercurial-scm.org/D2659

mercurial/templater.py

@@ -1326,7 +1326,7 b' def _flatten(thing):'
         # We can only hit this on Python 3, and it's here to guard
         # against infinite recursion.
         raise error.ProgrammingError('Mercurial IO including templates is done'
-                                     ' with bytes, not strings')
+                                     ' with bytes, not strings, got %r' % thing)
     elif thing is None:
         pass
     elif not util.safehasattr(thing, '__iter__'):