rebase -i: use diff plumbing instead of porcelain

When diff drivers are installed, calling "git diff <tree1>..<tree2>"
calls those drivers.  This borks the patch generation of rebase -i.
So use "git diff-tree -p" instead, which does not call diff drivers.

Noticed by Johannes Sixt.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-rebase--interactive.sh

@@ -78,7 +78,7 @@ mark_action_done () {
 
 make_patch () {
 	parent_sha1=$(git rev-parse --verify "$1"^ 2> /dev/null)
-	git diff "$parent_sha1".."$1" > "$DOTEST"/patch
+	git diff-tree -p "$parent_sha1".."$1" > "$DOTEST"/patch
 }
 
 die_with_patch () {
@@ -302,7 +302,7 @@ do_next () {
 	git update-ref -m "$message" $HEADNAME $NEWHEAD $OLDHEAD &&
 	git symbolic-ref HEAD $HEADNAME && {
 		test ! -f "$DOTEST"/verbose ||
-			git diff --stat $(cat "$DOTEST"/head)..HEAD
+			git diff-tree --stat $(cat "$DOTEST"/head)..HEAD
 	} &&
 	rm -rf "$DOTEST" &&
 	warn "Successfully rebased and updated $HEADNAME."