diff-tree: avoid lookup_unknown_object

We generally want to avoid lookup_unknown_object, because it
results in allocating more memory for the object than may be
strictly necessary.

In this case, it is used to check whether we have an
already-parsed object before calling parse_object, to save
us from reading the object from disk. Using lookup_object
would be fine for that purpose, but we can take it a step
further. Since this code was written, parse_object already
learned the "check lookup_object" optimization, so we can
simply call parse_object directly.

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

builtin/diff-tree.c

diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
index be6417d..dddd0f9 100644
--- a/builtin/diff-tree.c
+++ b/builtin/diff-tree.c
@@ -72,9 +72,7 @@
 	line[len-1] = 0;
 	if (get_sha1_hex(line, sha1))
 		return -1;
-	obj = lookup_unknown_object(sha1);
-	if (!obj || !obj->parsed)
-		obj = parse_object(sha1);
+	obj = parse_object(sha1);
 	if (!obj)
 		return -1;
 	if (obj->type == OBJ_COMMIT)