t3200: finish transitioning to the initial branch name main

In 56300ff (t3200: prepare for `main` being shorter than `master`,
2020-10-23) and in 66713e8 (tests: prepare aligned mentions of the
default branch name, 2020-10-23), we started to prepare t3200 for a new
world where `git init` uses the branch name `main` for the initial
branch.

We do not even have to wait for that new world: we can easily ensure
that that branch name is used, independent of the exact name `git init`
will give the initial branch, so let's do that.

This also lets us remove the `PREPARE_FOR_MAIN_BRANCH` prereq from three
test cases in that script.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho