patch-ids: document intentional const-casting in patch_id_neq()

The hashmap API requires the comparison function to take const pointers.
However, patch_id_neq() uses lazy evaluation to compute patch IDs on
demand. As established in b3dfeebb (rebase: avoid computing unnecessary
patch IDs, 2016-07-29), this avoids unnecessary work since not all
objects in the hashmap will eventually be compared.

Remove the ten-year-old "NEEDSWORK" comment and formally document
this intentional design trade-off.

Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

patch-ids.c

@@ -41,7 +41,14 @@ static int patch_id_neq(const void *cmpfn_data,
 			const struct hashmap_entry *entry_or_key,
 			const void *keydata UNUSED)
 {
-	/* NEEDSWORK: const correctness? */
+	/*
+	 * We drop the 'const' modifier here intentionally.
+	 *
+	 * Even though eptr and entry_or_key are const, we want to
+	 * lazily compute their .patch_id members; see b3dfeebb (rebase:
+	 * avoid computing unnecessary patch IDs, 2016-07-29). So we cast
+	 * the constness away with container_of().
+	 */
 	struct diff_options *opt = (void *)cmpfn_data;
 	struct patch_id *a, *b;