t/t3430: avoid undefined git diff behavior

The autosquash-and-exec test used "git diff HEAD^!" to mean
"git diff HEAD^ HEAD".  Use these directly instead of relying
on the undefined but actual-current behavior of "HEAD^!".

Signed-off-by: Chris Torek <chris.torek@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3430-rebase-merges.sh

@@ -420,7 +420,7 @@ test_expect_success 'with --autosquash and --exec' '
 	git commit --fixup B B.t &&
 	write_script show.sh <<-\EOF &&
 	subject="$(git show -s --format=%s HEAD)"
-	content="$(git diff HEAD^! | tail -n 1)"
+	content="$(git diff HEAD^ HEAD | tail -n 1)"
 	echo "$subject: $content"
 	EOF
 	test_tick &&