ref-filter: fix outdated comment on in_commit_list

The in_commit_list() method does not check the parents of
the candidate for containment in the list. Fix the comment
that incorrectly states that it does.

Reported-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1582,7 +1582,7 @@ static int in_commit_list(const struct commit_list *want, struct commit *c)
 }
 
 /*
- * Test whether the candidate or one of its parents is contained in the list.
+ * Test whether the candidate is contained in the list.
  * Do not recurse to find out, though, but return -1 if inconclusive.
  */
 static enum contains_result contains_test(struct commit *candidate,