t1400: use `main` as initial branch name

t1400: use `main` as initial branch name

In 3224b0f0bb7 (t1400: prepare for `main` being default branch name,
2020-10-23), we prepared t1400 for a time when the default initial
branch name would be `main`.

However, there is no need to wait that long: 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>