t5515: use `main` as the name of the main branch for testing (part 1)

t5515: use `main` as the name of the main branch for testing (part 1)

As part of the effort to change the default branch name to `main`, let's
prepare t5515.

In addition to adjusting the references to the branch name itself, this
also requires two commit hashes to be adjusted (actually four, as there
is a SHA-1 _and_ a SHA-256 of both).

That trick was performed by running

    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-*.sh

These commit hashes have been determined manually, of course, by running
the test after adjusting only the branch names, and then copying the
hashes from the log of the failed run.

Note: this patch only touches the t5515 script so far, not the
supporting material in t/t5515/. The resulting patch would have weighed
over 100kB and therefore the Git mailing list would have dropped it. The
files in t/t5515/ will be adjusted in the next two commits. As t5515
would fail without these adjustments, we temporarily skip it via the
`PREPARE_FOR_MAIN_BRANCH` prereq.

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