t7[5-9]*: adjust the references to the default branch name "main"

t7[5-9]*: adjust the references to the default branch name "main"

Excluding t7817, which is added in an unrelated patch series at the time
of writing, this adjusts t7[5-9]*. This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t7[5-9]*.sh)

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>