hash-ll, hashmap: move oidhash() to hash-ll

hash-ll, hashmap: move oidhash() to hash-ll

oidhash() was used by both hashmap and khash, which makes sense.
However, the location of this function in hashmap.[ch] meant that
khash.h had to depend upon hashmap.h, making people unfamiliar with
khash think that it was built upon hashmap.  (Or at least, I personally
was confused for a while about this in the past.)

Move this function to hash-ll, so that khash.h can stop depending upon
hashmap.h.

This has another benefit as well: it allows us to remove hashmap.h's
dependency on hash-ll.h.  While some callers of hashmap.h were making
use of oidhash, most were not, so this change provides another way to
reduce the number of includes.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>