fsck: check HAS_OBJ more consistently

There are two spots that call lookup_object() and assume
that a non-NULL result means we have the object:

  1. When we're checking the objects given to us by the user
     on the command line.

  2. When we're checking if a reflog entry is valid.

This generally follows fsck's mental model that we will have
looked at and loaded a "struct object" for each object in
the repository. But it misses one case: if another object
_mentioned_ an object, but we didn't actually parse it or
verify that it exists, it will still have a struct.

It's not clear if this is a triggerable bug or not.
Certainly the later parts of the reachability check need to
be careful of this, and do so by checking the HAS_OBJ flag.
But both of these steps happen before we start traversing,
so probably we won't have followed any links yet. Still,
it's easy enough to be defensive here.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fsck.c

@@ -398,7 +398,7 @@ static void fsck_handle_reflog_sha1(const char *refname, unsigned char *sha1,
 
 	if (!is_null_sha1(sha1)) {
 		obj = lookup_object(sha1);
-		if (obj) {
+		if (obj && (obj->flags & HAS_OBJ)) {
 			if (timestamp && name_objects)
 				add_decoration(fsck_walk_options.object_names,
 					obj,
@@ -755,7 +755,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 		if (!get_sha1(arg, sha1)) {
 			struct object *obj = lookup_object(sha1);
 
-			if (!obj) {
+			if (!obj || !(obj->flags & HAS_OBJ)) {
 				error("%s: object missing", sha1_to_hex(sha1));
 				errors_found |= ERROR_OBJECT;
 				continue;