t2*: adjust the references to the default branch name "main"

t2*: adjust the references to the default branch name "main"

Carefully excluding t2106, which sees independent development elsewhere
at the time of writing, we transition above-mentioned tests to the
default branch name `main`. This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t2*.sh &&
	   git checkout HEAD -- t2106\*)

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>