[PATCH] Teach diff-tree to report unmodified paths for -C option.

Linus, I have a bit of design issue that there is no way for the
diff-core layer to affect the behaviour of the caller.  It is
not too big a problem with others, but for diff-tree the effect
is prominent.

If you do this:

  ./git-rev-list HEAD | ./git-diff-tree -Sdiff-tree-helper --stdin -M

you will get many uninteresting headers from git-diff-tree,
until you hit something "interesting".  What I want to see in
this case is probably to omit the header and commit information
diff-tree usually gives while "pickaxe" filters everything out
(i.e. nothing interesting to report), and teach diff-tree to do
the header thing only when diff-core says there is something
interesting.  So far I haven't found a good way to do this.

Another useless comment.  For obvious reasons, there is nothing
we can do about the diff-helper to add "the other half of copy
detection information", because what it can tell diff-core is
limited to its input, which usually is just differences prepared
by somebody else, and it cannot know anything about unchanged
files.  When I started pushing '-p' flag to diff-tree family, I
remember that your reaction was neutral to moderately negative
("I'd tolerate, although I think it is redundant and you are not
even generating diff yourself anyway" as opposed to "That's just
great").  I think now you would thank me for shoving the diff
interface into them ;-).

I did not touch diff-tree for full-scale -C option in the last
series, but if you want to have it, it is easy.

This one is untested but it should just work (TM).

Signed-off-by: Junio C Hamano <redacted>
---
# - HEAD: Diff overhaul, adding the other half of copy detection.
# + (working tree)

diff --git a/diff-tree.c b/diff-tree.c
--- a/diff-tree.c
+++ b/diff-tree.c

@@ -117,7 +117,7 @@ static int compare_tree_entry(void *tree
 		show_file("+", tree2, size2, base);
 		return 1;
 	}
-	if (!memcmp(sha1, sha2, 20) && mode1 == mode2)
+	if (!memcmp(sha1, sha2, 20) && mode1 == mode2 && detect_rename < 2)
 		return 0;
 
 	/*