[PATCH 0/3] diff overhaul

Here are three patches to update diff we discussed earlier.

  [PATCH 1/3] Update mode-change strings in diff output.
  [PATCH 2/3] Rename git-diff-tree-helper to git-diff-helper.
  [PATCH 3/3] Rename git-diff-tree-helper to git-diff-helper (part 2).

The first one is to normalize the mode bits output to be more machine friendly, which we discussed.

The second commit records only renames for two files and there is no content change. The third one is to change the Makefiles, documentation, and also usage string in git-diff-helper itself.

I am deliverately doing the 2 and 3 in separate steps for two reasons. (1) to reduce the possibility of mistakes during the rename for myself; (2) people doing the git repository browser can use the commit trail for practicing their rename detection logic. So please keep these two steps separate when you commit.