oidset: refactor oidset_insert_from_set()

In a following commit, we will need to add all the oids from a set into
another set. In "list-objects-filter.c", there is already a static
function called add_all() to do that.

Let's rename this function oidset_insert_from_set() and move it into
oidset.{c,h} to make it generally available.

While at it, let's remove a useless `!= NULL`.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oidset.c

@@ -23,6 +23,16 @@ int oidset_insert(struct oidset *set, const struct object_id *oid)
 	return !added;
 }
 
+void oidset_insert_from_set(struct oidset *dest, struct oidset *src)
+{
+	struct oidset_iter iter;
+	struct object_id *src_oid;
+
+	oidset_iter_init(src, &iter);
+	while ((src_oid = oidset_iter_next(&iter)))
+		oidset_insert(dest, src_oid);
+}
+
 int oidset_remove(struct oidset *set, const struct object_id *oid)
 {
 	khiter_t pos = kh_get_oid_set(&set->set, *oid);