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

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>