t6[0-3]*: adjust the references to the default branch name "main"

t6[0-3]*: adjust the references to the default branch name "main"

Carefully excluding t6300, which sees independent development elsewhere
at the time of writing, we use `main` as the default branch name in
t6[0-3]*. This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t6[0-3]*.sh &&
	   git checkout HEAD -- t6300\*)

This allows us to define `GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main`
for those tests.

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