[PATCH] Make diff-cache and friends output more cg-patch friendly.

[PATCH] Make diff-cache and friends output more cg-patch friendly.

This changes the way the default arguments to diff are built when
diff-cache and friends are invoked with -p and there is no
GIT_EXTERNAL_DIFF environment variable.  It attempts to be more cg-patch
friendly by:

 - Showing diffs against /dev/null to denote added or removed
   files;

 - Showing file modes for existing files as a comment after the
   diff label.

Unfortunately with this change GIT_DIFF_CMD customization cannot
be supported easily anymore, so it has been dropped.
GIT_DIFF_OPTS customization to change diffs from unified to
context is still there, though.

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