t1007: factor out repeated setup

t1007: factor out repeated setup

We have a series of 3 CRLF tests that do exactly the same
(long) setup sequence. Let's pull it out into a common setup
test, which is shorter, more efficient, and will make it
easier to add new tests.

Note that we don't have to worry about cleaning up any of
the setup which was previously per-test; we call pop_repo
after the CRLF tests, which cleans up everything.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>