t5703: use `main` as initial branch name

t5703: use `main` as initial branch name

In 97cf8d50b59 (t5703: adjust a test case for the upcoming default
branch name, 2020-10-23), we prepared this test script for a world 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 one test case.

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