Teach git diff-tree --stdin to diff trees

When feeding trees on the command line, you can give exactly two
trees, not three nor one; --stdin now supports this "two tree" form on
its input, in addition to accepting lines with one or more commits.

When diffing trees (either specified on the command line or from the
standard input), the -s, -v, --pretty, --abbrev-commit, --encoding,
--no-commit-id, and --always options are ignored, since they do not
apply to trees; and the -m, -c, and --cc options are ignored since
they would be meaningful only with three or more trees, which is not
supported (yet).

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff-tree.txt

@@ -49,17 +49,22 @@ include::diff-options.txt[]
 --stdin::
 	When '--stdin' is specified, the command does not take
 	<tree-ish> arguments from the command line.  Instead, it
-	reads either one <commit> or a list of <commit>
-	separated with a single space from its standard input.
+	reads lines containing either two <tree>, one <commit>, or a
+	list of <commit> from its standard input.  (Use a single space
+	as separator.)
 +
+When two trees are given, it compares the first tree with the second.
 When a single commit is given, it compares the commit with its
 parents.  The remaining commits, when given, are used as if they are
 parents of the first commit.
 +
-The ID of the first (or only) commit, followed by a newline, is
-printed before the differences.
+When comparing two trees, the ID of both trees (separated by a space
+and terminated by a newline) is printed before the difference.  When
+comparing commits, the ID of the first (or only) commit, followed by a
+newline, is printed.
 +
-The following flags further affects its behavior.
+The following flags further affects the behavior when comparing
+commits (but not trees).
 
 -m::
 	By default, 'git-diff-tree --stdin' does not show

builtin-diff-tree.c

@@ -42,21 +42,46 @@ static int stdin_diff_commit(struct commit *commit, char *line, int len)
 	return log_tree_commit(&log_tree_opt, commit);
 }
 
+/* Diff two trees. */
+static int stdin_diff_trees(struct tree *tree1, char *line, int len)
+{
+	unsigned char sha1[20];
+	struct tree *tree2;
+	if (len != 82 || !isspace(line[40]) || get_sha1_hex(line + 41, sha1))
+		return error("Need exactly two trees, separated by a space");
+	tree2 = lookup_tree(sha1);
+	if (!tree2 || parse_tree(tree2))
+		return -1;
+	printf("%s %s\n", sha1_to_hex(tree1->object.sha1),
+			  sha1_to_hex(tree2->object.sha1));
+	diff_tree_sha1(tree1->object.sha1, tree2->object.sha1,
+		       "", &log_tree_opt.diffopt);
+	log_tree_diff_flush(&log_tree_opt);
+	return 0;
+}
+
 static int diff_tree_stdin(char *line)
 {
 	int len = strlen(line);
 	unsigned char sha1[20];
-	struct commit *commit;
+	struct object *obj;
 
 	if (!len || line[len-1] != '\n')
 		return -1;
 	line[len-1] = 0;
 	if (get_sha1_hex(line, sha1))
 		return -1;
-	commit = lookup_commit(sha1);
-	if (!commit || parse_commit(commit))
+	obj = lookup_object(sha1);
+	obj = obj ? obj : parse_object(sha1);
+	if (!obj)
 		return -1;
-	return stdin_diff_commit(commit, line, len);
+	if (obj->type == OBJ_COMMIT)
+		return stdin_diff_commit((struct commit *)obj, line, len);
+	if (obj->type == OBJ_TREE)
+		return stdin_diff_trees((struct tree *)obj, line, len);
+	error("Object %s is a %s, not a commit or tree",
+	      sha1_to_hex(sha1), typename(obj->type));
+	return -1;
 }
 
 static const char diff_tree_usage[] =