t3403: fix commit authorship

t3403: fix commit authorship

Setting GIT_AUTHOR_* when committing with --amend will only change the
author if we also pass --reset-author.  This commit is used in some
tests that ensure the author ident does not change when rebasing.
Creating this commit without changing the authorship meant that the
test would not catch regressions that caused rebase to discard the
original authorship information.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>