refs: move object peeling into "object.c"

Peeling an object has nothing to do with refs, but we still have the
code in "refs.c". Move it over into "object.c", which is a more natural
place to put it.

Ideally, we'd also move `peel_iterated_oid()` over into "object.c". But
this function is tied to the refs interfaces because it uses a global
ref iterator variable to optimize peeling when the iterator already has
the peeled object ID readily available.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -207,6 +207,27 @@ struct object *lookup_object_by_type(struct repository *r,
 	}
 }
 
+enum peel_status peel_object(const struct object_id *name, struct object_id *oid)
+{
+	struct object *o = lookup_unknown_object(the_repository, name);
+
+	if (o->type == OBJ_NONE) {
+		int type = oid_object_info(the_repository, name, NULL);
+		if (type < 0 || !object_as_type(o, type, 0))
+			return PEEL_INVALID;
+	}
+
+	if (o->type != OBJ_TAG)
+		return PEEL_NON_TAG;
+
+	o = deref_tag_noverify(o);
+	if (!o)
+		return PEEL_INVALID;
+
+	oidcpy(oid, &o->oid);
+	return PEEL_PEELED;
+}
+
 struct object *parse_object_buffer(struct repository *r, const struct object_id *oid, enum object_type type, unsigned long size, void *buffer, int *eaten_p)
 {
 	struct object *obj;