t5[0-4]*: adjust the references to the default branch name "main"

t5[0-4]*: adjust the references to the default branch name "main"

Carefully excluding t5310, which is developed independently of the
current patch series at the time of writing, we now use `main` as
default branch in t5[0-4]*. This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t5[0-4]*.sh &&
	   git checkout HEAD -- t5310\*)

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

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>