oidset: make oidset_size() an inline function

oidset: make oidset_size() an inline function

oidset_size() just reads a single word from memory and returns it.
Avoid the function call overhead for this trivial operation by turning
it into an inline function.

While we're at it, declare its parameter const to allow it to be used
on read-only oidsets.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>