t3201: finalize transitioning to using the branch name `main`

t3201: finalize transitioning to using the branch name `main`

In 66713e84e71 (tests: prepare aligned mentions of the default branch
name, 2020-10-23), we started that transition, trying to prepare for a
time when `git init` would use that name for the initial branch.

Even if that time has not arrived, we can complete the transition by
making the test script independent of the default branch name. This also
allows us to drop the `PREPARE_FOR_MAIN_BRANCH` prereq from one test
case.

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