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>

oidset.c

@@ -36,11 +36,6 @@ void oidset_clear(struct oidset *set)
 	oidset_init(set, 0);
 }
 
-int oidset_size(struct oidset *set)
-{
-	return kh_size(&set->set);
-}
-
 void oidset_parse_file(struct oidset *set, const char *path)
 {
 	oidset_parse_file_carefully(set, path, NULL, NULL);