t5515: use `main` as the name of the main branch for testing (conclusion)

t5515: use `main` as the name of the main branch for testing (conclusion)

In the previous three commits, We prepared the `t5515` script and the
files in `t/t5515/` for the upcoming change of the default branch name
to `main`. The changes were made over the course of three commits
because the overall patch would have been too big to send to the Git
mailing list for review.

Naturally, the test could not pass in the transitional stages and was
therefore disabled via the `PREPARE_FOR_MAIN_BRANCH` prereq. Now that
the transition is complete, we can re-enable it.

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