t5411: start using the default branch name "main"

t5411: start using the default branch name "main"

This is a straight-forward search-and-replace in the test script;
However, this is not yet complete because it requires many more
replacements in `t/t5411/`, too many for a single patch (the Git mailing
list rejects mails larger than 100kB). For that reason, we disable this
test script temporarily via the `PREPARE_FOR_MAIN_BRANCH` prereq.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>