[PATCH 2/3 v2] Teach git diff-tree --stdin to diff trees

["Karl Hasselström" <kha@treskal.com>]

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 -m, -s, -v, --pretty, --abbrev-commit,
--encoding, --no-commit-id, -c, --cc, and --always options are
ignored, since they do not apply to trees.

Signed-off-by: Karl Hasselström <kha@treskal.com>

---

With updated commit message. (And I checked -- your earlier doc patch
_is_ obsoleted by this patch, so I saw no need to change anything
else.)

 Documentation/git-diff-tree.txt |   14 +++++++++-----
 builtin-diff-tree.c             |   35 +++++++++++++++++++++++++++++++----
 2 files changed, 40 insertions(+), 9 deletions(-)


diff --git a/Documentation/git-diff-tree.txt b/Documentation/git-diff-tree.txt
index 1fdf20d..0b1ade8 100644
--- a/Documentation/git-diff-tree.txt
+++ b/Documentation/git-diff-tree.txt
@@ -49,13 +49,17 @@ 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 a single commit is given on one line of such input, it compares
-the commit with its parents.  The following flags further affects its
-behavior.  The remaining commits, when given, are used as if they are
+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 following flags further affects the behavior when comparing
+commits (but not trees).
 
 -m::
 	By default, 'git-diff-tree --stdin' does not show
diff --git a/builtin-diff-tree.c b/builtin-diff-tree.c
index ebbd631..0bdb1cf 100644
--- a/builtin-diff-tree.c
+++ b/builtin-diff-tree.c
@@ -42,21 +42,48 @@ 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)) {
+		error("Need precisely two trees, separated by one space");
+		return -1;
+	}
+	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[] =