rebase: use test_commit helper in setup

rebase: use test_commit helper in setup

To prepare for the next commit that will test rebase with oids instead
of branch names, update the rebase setup test to add a couple of tags we
can use. This uses the test_commit helper so we can replace some lines
that add a commit manually.

Setting logAllRefUpdates is not necessary because it's on by default for
repositories with a working tree.

Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>