t5411: start adjusting the support files for init.defaultBranch=main

t5411: start adjusting the support files for init.defaultBranch=main

This trick was performed via

	$ sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t/t5411/test-00[3-5]*

We do not convert the files in `t/t5411/` in one go because the patch
would be too big (mails larger than 100kB are rejected by the Git
mailing list). Instead, we start with roughly half of the support files.

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