t9902: use main as initial branch name

In 8164360 (t9902: prepare a test for the upcoming default branch
name, 2020-10-23), we started adjusting this test script for the default
initial branch name changing to `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 one test case.

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

Author: dscho