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>
diff --git a/oidset.h b/oidset.h
index 5346563..80c3a34 100644
--- a/oidset.h
+++ b/oidset.h
@@ -56,6 +56,11 @@ int oidset_insert(struct oidset *set, const struct object_id *oid);
 int oidset_remove(struct oidset *set, const struct object_id *oid);
 
 /**
+ * Returns the number of oids in the set.
+ */
+int oidset_size(struct oidset *set);
+
+/**
  * Remove all entries from the oidset, freeing any resources associated with
  * it.
  */