rebase -i: fix squashing corner case

When squashing, rebase -i did not prevent fast forwards.  This could
happen when picking some other commit than the first one, and then
squashing the first commit.  So do not allow fast forwards when
squashing.

Noticed by Johannes Sixt.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -96,13 +96,14 @@ die_abort () {
 }
 
 pick_one () {
-	case "$1" in -n) sha1=$2 ;; *) sha1=$1 ;; esac
+	no_ff=
+	case "$1" in -n) sha1=$2; no_ff=t ;; *) sha1=$1 ;; esac
 	output git rev-parse --verify $sha1 || die "Invalid commit name: $sha1"
 	test -d "$REWRITTEN" &&
 		pick_one_preserving_merges "$@" && return
 	parent_sha1=$(git rev-parse --verify $sha1^ 2>/dev/null)
 	current_sha1=$(git rev-parse --verify HEAD)
-	if test $current_sha1 = $parent_sha1; then
+	if test $no_ff$current_sha1 = $parent_sha1; then
 		output git reset --hard $sha1
 		test "a$1" = a-n && output git reset --soft $current_sha1
 		sha1=$(git rev-parse --short $sha1)

t/t3404-rebase-interactive.sh

@@ -264,6 +264,27 @@ test_expect_success 'interrupted squash works as expected' '
 	test $one = $(git rev-parse HEAD~2)
 '
 
+test_expect_success 'interrupted squash works as expected (case 2)' '
+	for n in one two three four
+	do
+		echo $n >> conflict &&
+		git add conflict &&
+		git commit -m $n
+	done &&
+	one=$(git rev-parse HEAD~3) &&
+	! FAKE_LINES="3 squash 1 2" git rebase -i HEAD~3 &&
+	(echo one; echo four) > conflict &&
+	git add conflict &&
+	! git rebase --continue &&
+	(echo one; echo two; echo four) > conflict &&
+	git add conflict &&
+	! git rebase --continue &&
+	echo resolved > conflict &&
+	git add conflict &&
+	git rebase --continue &&
+	test $one = $(git rev-parse HEAD~2)
+'
+
 test_expect_success 'ignore patch if in upstream' '
 	HEAD=$(git rev-parse HEAD) &&
 	git checkout -b has-cherry-picked HEAD^ &&