Update docs and usages regarding '-r' recursive option for git-diff-tree.
Remove '-r' from common diff options, mention it only for git-diff-tree.
Remove one extraneous use of '-r' with git-diff-files in get-merge.sh.
Sync the synopsis and usage string for git-diff-tree.
Signed-off-by: Chris Shoemaker <c.shoemaker at cox.net> Signed-off-by: Junio C Hamano <junkio@cox.net>