logcmdutil: add a helpful assertion to catch mistyped templates early

This would have made a defect in test-notify.t much easier to figure out.

Differential Revision: https://phab.mercurial-scm.org/D5097

mercurial/logcmdutil.py

@@ -460,6 +460,8 b' class changesettemplater(changesetprinte'
                 self.footer = self.t.render(self._parts['footer'], props)
 
 def templatespec(tmpl, mapfile):
+    if pycompat.ispy3:
+        assert not isinstance(tmpl, str), 'tmpl must not be a str'
     if mapfile:
         return formatter.templatespec('changeset', tmpl, mapfile)
     else: