t55[23]*: adjust the references to the default branch name "main"

t55[23]*: adjust the references to the default branch name "main"

Carefully excluding t5526, which sees independent development elsewhere
at the time of writing, we use `main` as the default branch name in
t55[23]*. This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -e 's/naster/nain/g' -- \
		t55[23]*.sh &&
	   git checkout HEAD -- t5526\*)

Note that t5533 contains a variation of the name `master` (`naster`)
that we rename here, too.

This commit allows us to define
`GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main` for that range of tests.

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