revlog: delete references to deleted nullid sentinel value

We stopped keeping the nullid in the index in 781b2720d2ac (index:
don't include nullid in len(), 2018-07-20). I forgot to update some
comments about it.

Differential Revision: https://phab.mercurial-scm.org/D7366

mercurial/cext/revlog.c

@@ -62,10 +62,9 b' typedef struct {'
  * This class has two behaviors.
  *
  * When used in a list-like way (with integer keys), we decode an
- * entry in a RevlogNG index file on demand. Our last entry is a
- * sentinel, always a nullid.  We have limited support for
+ * entry in a RevlogNG index file on demand. We have limited support for
  * integer-keyed insert and delete, only at elements right before the
- * sentinel.
+ * end.
  *
  * With string keys, we lazily perform a reverse mapping from node to
  * rev, using a base-16 trie.
@@ -2466,7 +2465,7 b' static void index_invalidate_added(index'
 
 /*
  * Delete a numeric range of revs, which must be at the end of the
- * range, but exclude the sentinel nullid entry.
+ * range.
  */
 static int index_slice_del(indexObject *self, PyObject *item)
 {