Documentation changes to recursive option for git-diff-tree

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>

git-merge.sh

@@ -25,7 +25,7 @@ dropsave() {
 
 savestate() {
 	# Stash away any local modifications.
-	git-diff-index -r -z --name-only $head |
+	git-diff-index -z --name-only $head |
 	cpio -0 -o >"$GIT_DIR/MERGE_SAVE"
 }