t3437: use --fixup with options to create amend! commit

We taught `git commit --fixup` to create "amend!" commit. Let's also
update the tests and use it to setup the rebase tests.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: charvi-077

t/t3437-rebase-fixup-options.sh

@@ -72,40 +72,16 @@ test_expect_success 'setup' '
 	git commit --fixup=HEAD -a &&
 	git tag B1 &&
 	test_tick &&
-	git commit --allow-empty -F - <<-EOF &&
-	amend! B
-	$EMPTY
-	B
-	$EMPTY
-	edited 1
-	EOF
+	FAKE_COMMIT_AMEND="edited 1" git commit --fixup=reword:B &&
 	test_tick &&
-	git commit --allow-empty -F - <<-EOF &&
-	amend! amend! B
-	$EMPTY
-	B
-	$EMPTY
-	edited 1
-	$EMPTY
-	edited 2
-	EOF
+	FAKE_COMMIT_AMEND="edited 2" git commit --fixup=reword:HEAD &&
 	echo B2 >B &&
 	test_tick &&
 	FAKE_COMMIT_AMEND="edited squash" git commit --squash=HEAD -a &&
 	git tag B2 &&
 	echo B3 >B &&
 	test_tick &&
-	git commit -a -F - <<-EOF &&
-	amend! amend! amend! B
-	$EMPTY
-	B
-	$EMPTY
-	edited 1
-	$EMPTY
-	edited 2
-	$EMPTY
-	edited 3
-	EOF
+	FAKE_COMMIT_AMEND="edited 3" git commit -a --fixup=amend:HEAD^ &&
 	git tag B3 &&
 
 	GIT_AUTHOR_NAME="Rebase Author" &&