describe: fix accidental oid/hash type-punning

describe: fix accidental oid/hash type-punning

The find_commit_name() function passes an object_id.hash as the key of a
hashmap. That ends up in commit_name_neq(), which then feeds it to
oideq(). Which means we should actually be the whole "struct object_id".

It works anyway because pointers to the two are interchangeable. And
because we're going through a layer of void pointers, the compiler
doesn't notice the type mismatch.

But it's worth cleaning up (especially since once we switch away from
sha1hash() on the same line, accessing the hash member will look doubly
out of place).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>