rebase -i -p: handle index and workdir correctly

'git rebase -i -p' forgot to update the index and working directory
during fast forwards.  Fix this.  Makes 'GIT_EDITOR=true rebase -i -p
<ancestor>' a no-op again.

Also, it attempted to do a fast forward even if it was instructed not
to commit (via -n).  Fall back to the cherry-pick code path and let
that handle the issue for us.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>

git-rebase--interactive.sh

@@ -145,7 +145,16 @@ pick_one () {
 }
 
 pick_one_preserving_merges () {
-	case "$1" in -n) sha1=$2 ;; *) sha1=$1 ;; esac
+	fast_forward=t
+	case "$1" in
+	-n)
+		fast_forward=f
+		sha1=$2
+		;;
+	*)
+		sha1=$1
+		;;
+	esac
 	sha1=$(git rev-parse $sha1)
 
 	if test -f "$DOTEST"/current-commit
@@ -157,7 +166,6 @@ pick_one_preserving_merges () {
 	fi
 
 	# rewrite parents; if none were rewritten, we can fast-forward.
-	fast_forward=t
 	preserve=t
 	new_parents=
 	for p in $(git rev-list --parents -1 $sha1 | cut -d' ' -f2-)
@@ -182,6 +190,8 @@ pick_one_preserving_merges () {
 	t)
 		output warn "Fast forward to $sha1"
 		test $preserve = f || echo $sha1 > "$REWRITTEN"/$sha1
+		output git reset --hard $sha1 ||
+			die "Cannot fast forward to $sha1"
 		;;
 	f)
 		test "a$1" = a-n && die "Refusing to squash a merge: $sha1"

t/t3404-rebase-interactive.sh

@@ -202,6 +202,9 @@ test_expect_success 'retain authorship when squashing' '
 test_expect_success '-p handles "no changes" gracefully' '
 	HEAD=$(git rev-parse HEAD) &&
 	git rebase -i -p HEAD^ &&
+	git update-index --refresh &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD -- &&
 	test $HEAD = $(git rev-parse HEAD)
 '
 
@@ -235,6 +238,9 @@ test_expect_success 'preserve merges with -p' '
 	git checkout -b to-be-rebased &&
 	test_tick &&
 	git rebase -i -p --onto branch1 master &&
+	git update-index --refresh &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD -- &&
 	test $(git rev-parse HEAD~6) = $(git rev-parse branch1) &&
 	test $(git rev-parse HEAD~4^2) = $(git rev-parse to-be-preserved) &&
 	test $(git rev-parse HEAD^^2^) = $(git rev-parse HEAD^^^) &&