[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 <junkio@cox.net>
---
# - HEAD: Diff overhaul, adding the other half of copy detection.
# + (working tree)