khash: rename kh_oid_t to kh_oid_set

khash: rename kh_oid_t to kh_oid_set

khash lets us define a hash as either a map or a set (i.e., with no
"value" type). For the oid maps we define, "oid" is the set and
"oid_map" is the map. As the bug in the previous commit shows, it's easy
to pick the wrong one.

So let's make the names more distinct: "oid_set" and "oid_map".

An alternative naming scheme would be to actually name the type after
the key/value types. So e.g., "oid" _would_ be the set, since it has no
value type. And "oid_map" would become "oid_void" or similar (and
"oid_pos" becomes "oid_int"). That's better in some ways: it's more
regular, and a given map type can be more reasily reused in multiple
contexts (e.g., something storing an "int" that isn't a "pos"). But it's
also slightly less descriptive.

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