patch-ids.c: use hashmap correctly

patch-ids.c: use hashmap correctly

As alluded to in the previous patch, the code in patch-ids.c is
using the hashmaps API wrong.

Luckily we do not have a bug, as all hashmap functionality that we use
here (hashmap_get) passes through the keydata.  If hashmap_get_next were
to be used, a bug would occur as that passes NULL for the key_data.

So instead use the hashmap API correctly and provide the caller required
data in the compare function via the first argument that always gets
passed and was setup via the hashmap_init function.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>