t3415: test fixup with wrapped oneline

The `git commit --fixup` command unwraps wrapped onelines when
constructing the commit message, without wrapping the result.

We need to make sure that `git rebase --autosquash` keeps handling such
cases correctly, in particular since we are about to move the autosquash
handling into the rebase--helper.

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

t/t3415-rebase-autosquash.sh

@@ -316,4 +316,18 @@ test_expect_success 'extra spaces after fixup!' '
 	test $base = $parent
 '
 
+test_expect_success 'wrapped original subject' '
+	if test -d .git/rebase-merge; then git rebase --abort; fi &&
+	base=$(git rev-parse HEAD) &&
+	echo "wrapped subject" >wrapped &&
+	git add wrapped &&
+	test_tick &&
+	git commit --allow-empty -m "$(printf "To\nfixup")" &&
+	test_tick &&
+	git commit --allow-empty -m "fixup! To fixup" &&
+	git rebase -i --autosquash --keep-empty HEAD~2 &&
+	parent=$(git rev-parse HEAD^) &&
+	test $base = $parent
+'
+
 test_done