t/t3437: fixup the test 'multiple fixup -c opens editor once'

t/t3437: fixup the test 'multiple fixup -c opens editor once'

In the test, FAKE_COMMIT_MESSAGE replaces the commit message each
time it is invoked so there will be only one instance of "Modified-A3"
no matter how many times we invoke the editor. Let's fix this and use
FAKE_COMMIT_AMEND instead so that it adds "Modified-A3" once for each
time the editor is invoked.

This patch also removes the check for counting the number of
"Modified-A3" lines and instead compares the whole message to check
that the commenting code works correctly for 'fixup -c' as well as
'fixup -C'.

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>