t5403: use test_cmp for post-checkout argument checks

t5403: use test_cmp for post-checkout argument checks

Update check_post_checkout and the post-checkout hook implementation to
use test_cmp instead of individual test commands. This provides better
error messages when tests fail, making it easier to debug which specific
argument (old ref, new ref, or flag) was incorrect.

The hook now outputs in key=value format which test_cmp can display
clearly when there's a mismatch.

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>