t/t3437: simplify and document the test helpers

t/t3437: simplify and document the test helpers

Let's simplify the test_commit_message() helper function and add
comments to the function.

This patch also document the working of 'fixup -C' with "amend!" in the
test-description.

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