t0008: don't rely on default ".git/info/exclude"

t0008: don't rely on default ".git/info/exclude"

Change a test added in 368aa52952d (add git-check-ignore sub-command,
2013-01-06) to clobber .git/info/exclude rather than append to
it.

These tests would break if the "templates/info--exclude" file added in
d3af621b147 (Redo the templates generation and installation.,
2005-08-06) wasn't exactly 6 lines (of only comments).

Let's instead clobber the default .git/info/excludes file, and test
only our own expected content. This is not strictly needed for
anything in this series, but is a good cleanup while we're at it.

As discussed in the preceding commit a lot of things depend on the
"info" directory being created, but this was the only test that relied
on the specific content in the "templates/info--exclude" file.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>