tests: prepare aligned mentions of the default branch name

tests: prepare aligned mentions of the default branch name

In some tests, the default branch name is part of aligned output. As we
want to change the default branch name to `main`, which is two
characters shorter than the old default branch name, we will have to
adjust those tests.

Since we use the original default branch name until the entire test
suite has been adjusted accordingly, the touched test cases need to be
guarded by a prereq (that is so far disabled so that they are skipped
for now).

The test cases that depend on those test cases that are newly guarded by
that prereq naturally have to be guarded, too.

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