oidset: introduce 'oidset_size'

Occasionally, it may be useful for callers to know the number of object
IDs in an oidset. Right now, the only way to compute this is to call
'kh_size' on the internal 'kh_set_oid_t'.

Similar to how we wrap other 'kh_*' functions over the 'oidset' type,
let's allow callers to compute this value by introducing 'oidset_size'.

We will add its first caller in the subsequent commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oidset.c

@@ -36,6 +36,11 @@ 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)
 {
 	FILE *fp;