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