doc/git-log: move "-t" into diff-options list

doc/git-log: move "-t" into diff-options list

The "-t" option is infrequently used; it doesn't deserve a spot near the
top of the options list. Let's push it down into the diff-options
include, near the definition of --raw.

We'll protect it with a git-log ifdef, since it doesn't make any sense
for non-tree diff commands. Note that this means it also shows up in
git-show, but that's a good thing; it applies equally well there.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>