templater: explode if we try to emit a str

Without this if branch, we infinitely recurse in _flatten, which is
very confusing. Something in an hgweb template is trying to write out
a string instead of a bytes on Python 3, and this at least makes it
crash politely.

Differential Revision: https://phab.mercurial-scm.org/D1088

mercurial/templater.py

@@ -1240,6 +1240,11 b' def _flatten(thing):'
     thing = templatekw.unwraphybrid(thing)
     if isinstance(thing, bytes):
         yield thing
+    elif isinstance(thing, str):
+        # 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')
     elif thing is None:
         pass
     elif not util.safehasattr(thing, '__iter__'):