hard-code the empty tree object

Now any commands may reference the empty tree object by its
sha1 (4b825dc642cb6eb9a060e54bf8d69288fbee4904). This is
useful for showing some diffs, especially for initial
commits.

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

sha1_file.c

@@ -1845,6 +1845,15 @@ static struct cached_object {
 } *cached_objects;
 static int cached_object_nr, cached_object_alloc;
 
+static struct cached_object empty_tree = {
+	/* empty tree sha1: 4b825dc642cb6eb9a060e54bf8d69288fbee4904 */
+	"\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60"
+	"\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04",
+	OBJ_TREE,
+	"",
+	0
+};
+
 static struct cached_object *find_cached_object(const unsigned char *sha1)
 {
 	int i;
@@ -1854,6 +1863,8 @@ static struct cached_object *find_cached_object(const unsigned char *sha1)
 		if (!hashcmp(co->sha1, sha1))
 			return co;
 	}
+	if (!hashcmp(sha1, empty_tree.sha1))
+		return &empty_tree;
 	return NULL;
 }