templatefilters: avoid infinite recursion bug in stringify

This doesn't ever happen on Python 2, but it's been a persistent pain
on Python 3. Adding this helped produce some of my upcoming Python 3
fixes.

Differential Revision: https://phab.mercurial-scm.org/D2553

mercurial/templatefilters.py

@@ -376,6 +376,12 b' def stringify(thing):'
     """
     thing = templatekw.unwraphybrid(thing)
     if util.safehasattr(thing, '__iter__') and not isinstance(thing, bytes):
+        if isinstance(thing, str):
+            # This is only reachable on Python 3 (otherwise
+            # isinstance(thing, bytes) would have been true), and is
+            # here to prevent infinite recursion bugs on Python 3.
+            raise error.ProgrammingError(
+                'stringify got unexpected unicode string: %r' % thing)
         return "".join([stringify(t) for t in thing if t is not None])
     if thing is None:
         return ""