diff-lib: use ce_mode_from_stat() rather than messing with modes manually

diff-lib: use ce_mode_from_stat() rather than messing with modes manually

The diff helpers used to do the magic mode canonicalization and all the
other special mode handling by hand ("trust executable bit" and "has
symlink support" handling).

That's bogus. Use "ce_mode_from_stat()" that does this all for us.

This is also going to be required when we add support for links to other
git repositories.

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