[PATCH] diff overhaul

This cleans up the way calls are made into the diff core from diff-tree
family and diff-helper.  Earlier, these programs had "if
(generating_patch)" sprinkled all over the place, but those ugliness are
gone and handled uniformly from the diff core, even when not generating
patch format.

This also allowed diff-cache and diff-files to acquire -R
(reverse) option to generate diff in reverse.  Users of
diff-tree can swap two trees easily so I did not add -R there.

[ Linus' note: I'll add -R to "diff-tree" too, since a "commit
  diff" doesn't have another tree to switch around: the other
  tree is always the parent(s) of the commit ]

Also -M<digits-as-mantissa> suggestion made by Linus has been
implemented.

Documentation updates are also included.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

t/t2002-checkout-cache-u.sh

@@ -29,3 +29,5 @@ rm -f path0 &&
 git-read-tree $t &&
 git-checkout-cache -u -f -a &&
 git-diff-files | diff - /dev/null'
+
+test_done