fileset: raise ProgrammingError for bad existing() calls

And glob out Py2/3 difference.

mercurial/fileset.py

@@ -566,7 +566,8 b' class matchctx(object):'
     def filter(self, files):
         return [f for f in files if f in self.subset]
     def existing(self):
-        assert self._existingenabled, 'unexpected existing() invocation'
+        if not self._existingenabled:
+            raise error.ProgrammingError('unexpected existing() invocation')
         if self._status is not None:
             removed = set(self._status[3])
             unknown = set(self._status[4] + self._status[5])

tests/test-fileset.t

@@ -446,7 +446,7 b" Test detection of unintentional 'matchct"
   > EOF
 
   $ fileset 'existingcaller()' 2>&1 | tail -1
-  AssertionError: unexpected existing() invocation
+  *ProgrammingError: *unexpected existing() invocation* (glob)
 
 Test 'revs(...)'
 ================