khash: rename oid helper functions

khash: rename oid helper functions

For use in object_id hash tables, we have oid_hash() and oid_equal().
But these are confusingly similar to the existing oideq() and the
oidhash() we plan to add to replace sha1hash().

The big difference from those functions is that rather than accepting a
const pointer to the "struct object_id", we take the arguments by value
(which is a khash internal convention). So let's make that obvious by
calling them oidhash_by_value() and oideq_by_value().

Those names are fairly horrendous to type, but we rarely need to do so;
they are passed to the khash implementation macro and then only used
internally. Callers get to use the nice kh_put_oid_map(), etc.

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