Fix git-diff-tree --stdin

140b378 (Teach git diff-tree --stdin to diff trees, 2008-08-10) broke the
more important case of reading series of commits to filter ones that touch
given pathspecs.

Noticed by Mark Levedahl, running "gitk ec3a4ba" and trying to focus on
commits that touch "t/" directory.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff-tree.c

@@ -71,8 +71,9 @@ static int diff_tree_stdin(char *line)
 	line[len-1] = 0;
 	if (get_sha1_hex(line, sha1))
 		return -1;
-	obj = lookup_object(sha1);
-	obj = obj ? obj : parse_object(sha1);
+	obj = lookup_unknown_object(sha1);
+	if (!obj || !obj->parsed)
+		obj = parse_object(sha1);
 	if (!obj)
 		return -1;
 	if (obj->type == OBJ_COMMIT)

object.h

@@ -41,7 +41,18 @@ extern int type_from_string(const char *str);
 extern unsigned int get_max_object_index(void);
 extern struct object *get_indexed_object(unsigned int);
 
-/** Internal only **/
+/*
+ * This can be used to see if we have heard of the object before, but
+ * it can return "yes we have, and here is a half-initialised object"
+ * for an object that we haven't loaded/parsed yet.
+ *
+ * When parsing a commit to create an in-core commit object, its
+ * parents list holds commit objects that represent its parents, but
+ * they are expected to be lazily initialized and do not know what
+ * their trees or parents are yet.  When this function returns such a
+ * half-initialised objects, the caller is expected to initialize them
+ * by calling parse_object() on them.
+ */
 struct object *lookup_object(const unsigned char *sha1);
 
 extern void *create_object(const unsigned char *sha1, int type, void *obj);