t5510: use main as initial branch name

In 66713e8 (tests: prepare aligned mentions of the default branch
name, 2020-10-23), we prepared this test script for a time when the
default initial branch name would be `main`.

However, there is no need to wait for that: let's adjust the test script
to stop relying on a specific initial branch name by setting it
explicitly. This allows us to drop the `PREPARE_FOR_MAIN_BRANCH` prereq
from two test cases.

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

Author: dscho