t5515: use `main` as the name of the main branch for testing (part 3)

t5515: use `main` as the name of the main branch for testing (part 3)

In the previous two commits, We just started preparing the `t5515` script
and part of `t/t5515/` for the upcoming change of the default
branch name to `main`. This patch adjusts the remainder of the supporting
material in `t/t5515/` (the patch adjusting all of `t/t5515/` would have
weighed more than 100kB and therefore not made it to the Git mailing
list for review).

Similar to what we did for the `t5515` script itself in the previous
commit, this patch was generated via:

    sed -i -e 's/master/main/g' -e 's/Master/Main/g' \
        -e 's/6c9dec2b923228c9ff994c6cfe4ae16c12408dc5/ecf3b3627b498bdcb735cc4343bf165f76964e9a/g' \
	-e 's/8521c3072461fcfe8f32d67f95cc6e6b832a2db2fa29769ffc788bce85ebcd75/fff666109892bb4b1c80cd1649d2d8762a0663db8b5d46c8be98360b64fbba5f/g' \
	-e 's/754b754407bf032e9a2f9d5a9ad05ca79a6b228f/b4ab76b1a01ea602209932134a44f1e6bd610832/g' \
	-e 's/6c7abaea8a6d8ef4d89877e68462758dc6774690fbbbb0e6d7dd57415c9abde0/380ebae0113f877ce46fcdf39d5bc33e4dc0928db5c5a4d5fdc78381c4d55ae3/g' \
	-- t/t5515/refs.*

In addition to that, we need to adjust some file _names_ in `t/t5515/`
because they encode the branch name:

    eval "$(git ls-files t/t5515/refs.\* | sed -n \
	-e 's/\(.*\)master\(.*\)/git mv & \1main\2;/p')"

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