patch, 4 partsTeach diff_tree_sha1() to accept NULL sha1 for empty trees

diff --git a/tree-diff.c b/tree-diff.c
index f7b3ade..f438478 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -287,14 +287,10 @@ int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const cha
 	unsigned long size1, size2;
 	int retval;
 
-	tree1 = read_object_with_reference(old, tree_type, &size1, NULL);
-	if (!tree1)
-		die("unable to read source tree (%s)", sha1_to_hex(old));
-	tree2 = read_object_with_reference(new, tree_type, &size2, NULL);
-	if (!tree2)
-		die("unable to read destination tree (%s)", sha1_to_hex(new));
-	init_tree_desc(&t1, tree1, size1);
-	init_tree_desc(&t2, tree2, size2);
+	tree1 = fill_tree_descriptor(&t1, old);
+	tree2 = fill_tree_descriptor(&t2, new);
+	size1 = t1.size;
+	size2 = t2.size;
 	retval = diff_tree(&t1, &t2, base, opt);
 	if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && diff_might_be_rename()) {
 		init_tree_desc(&t1, tree1, size1);

diff --git a/tree-diff.c b/tree-diff.c
index f438478..6d82a3f 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -304,18 +304,5 @@ int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const cha
 
 int diff_root_tree_sha1(const unsigned char *new, const char *base, struct diff_options *opt)
 {
-	int retval;
-	void *tree;
-	unsigned long size;
-	struct tree_desc empty, real;
-
-	tree = read_object_with_reference(new, tree_type, &size, NULL);
-	if (!tree)
-		die("unable to read root tree (%s)", sha1_to_hex(new));
-	init_tree_desc(&real, tree, size);
-
-	init_tree_desc(&empty, "", 0);
-	retval = diff_tree(&empty, &real, base, opt);
-	free(tree);
-	return retval;
+	return diff_tree_sha1(NULL, new, base, opt);
 }

diff --git a/line-log.c b/line-log.c
index 717638b..1500101 100644
--- a/line-log.c
+++ b/line-log.c
@@ -766,16 +766,6 @@ void line_log_init(struct rev_info *rev, const char *prefix, struct string_list
 	}
 }
 
-static void load_tree_desc(struct tree_desc *desc, void **tree,
-			   const unsigned char *sha1)
-{
-	unsigned long size;
-	*tree = read_object_with_reference(sha1, tree_type, &size, NULL);
-	if (!*tree)
-		die("Unable to read tree (%s)", sha1_to_hex(sha1));
-	init_tree_desc(desc, *tree, size);
-}
-
 static int count_parents(struct commit *commit)
 {
 	struct commit_list *parents = commit->parents;
@@ -842,18 +832,11 @@ static void queue_diffs(struct line_log_data *range,
 			struct diff_queue_struct *queue,
 			struct commit *commit, struct commit *parent)
 {
-	void *tree1 = NULL, *tree2 = NULL;
-	struct tree_desc desc1, desc2;
-
 	assert(commit);
-	load_tree_desc(&desc2, &tree2, commit->tree->object.sha1);
-	if (parent)
-		load_tree_desc(&desc1, &tree1, parent->tree->object.sha1);
-	else
-		init_tree_desc(&desc1, "", 0);
 
 	DIFF_QUEUE_CLEAR(&diff_queued_diff);
-	diff_tree(&desc1, &desc2, "", opt);
+	diff_tree_sha1(parent ? parent->tree->object.sha1 : NULL,
+			commit->tree->object.sha1, "", opt);
 	if (opt->detect_rename) {
 		filter_diffs_for_paths(range, 1);
 		if (diff_might_be_rename())
@@ -861,11 +844,6 @@ static void queue_diffs(struct line_log_data *range,
 		filter_diffs_for_paths(range, 0);
 	}
 	move_diff_queue(queue, &diff_queued_diff);
-
-	if (tree1)
-		free(tree1);
-	if (tree2)
-		free(tree2);
 }
 
 static char *get_nth_line(long line, unsigned long *ends, void *data)

diff --git a/revision.c b/revision.c
index 082dae6..bd027bc 100644
--- a/revision.c
+++ b/revision.c
@@ -497,24 +497,14 @@ static int rev_compare_tree(struct rev_info *revs,
 static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit)
 {
 	int retval;
-	void *tree;
-	unsigned long size;
-	struct tree_desc empty, real;
 	struct tree *t1 = commit->tree;
 
 	if (!t1)
 		return 0;
 
-	tree = read_object_with_reference(t1->object.sha1, tree_type, &size, NULL);
-	if (!tree)
-		return 0;
-	init_tree_desc(&real, tree, size);
-	init_tree_desc(&empty, "", 0);
-
 	tree_difference = REV_TREE_SAME;
 	DIFF_OPT_CLR(&revs->pruning, HAS_CHANGES);
-	retval = diff_tree(&empty, &real, "", &revs->pruning);
-	free(tree);
+	retval = diff_tree_sha1(NULL, t1->object.sha1, "", &revs->pruning);
 
 	return retval >= 0 && (tree_difference == REV_TREE_SAME);
 }