[PATCH] diff: Fix docs and add -O to diff-helper.

This patch updates diff documentation and usage strings:

 - clarify the semantics of -R.  It is not "output in reverse";
   rather, it is "I will feed diff backwards".  Semantically
   they are different when -C is involved.

 - describe -O in usage strings of diff-* brothers.  It was
   implemented, documented but not described in usage text.

Also it adds -O to diff-helper.  Like -S (and unlike -M/-C/-B),
this option can work on sanitized diff-raw output produced by
the diff-* brothers.  While we are at it, the call it makes to
diffcore is cleaned up to use the diffcore_std() like everybody
else, and the declaration for the low level diffcore routines
are moved from diff.h (public) to diffcore.h (private between
diff.c and diffcore backends).

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

diff-files.c

@@ -7,7 +7,7 @@
 #include "diff.h"
 
 static const char *diff_files_usage =
-"git-diff-files [-p] [-q] [-r] [-z] [-M] [-C] [-R] [-S<string>] [paths...]";
+"git-diff-files [-p] [-q] [-r] [-z] [-M] [-C] [-R] [-S<string>] [-O<orderfile>] [paths...]";
 
 static int diff_output_format = DIFF_FORMAT_HUMAN;
 static int detect_rename = 0;