t0008: improve test cleanup to fix failing test

t0008: improve test cleanup to fix failing test

The "large exclude file ignored in tree" test fails. This is due to an
additional warning message that is generated in the test. "warning:
unable to access 'subdir/.gitignore': Too many levels of symbolic
links", the extra warning that is not supposed to be there, happens
because of some leftover files left by previous tests.

To fix this we improve cleanup on "symlinks not respected in-tree", and
because the tests in t0008 in general have poor cleanup, at the start of
"large exclude file ignored in tree" we search for any leftover
.gitignore and remove them before starting the test.

Improve post-test cleanup and add pre-test cleanup to make sure that we
have a workable environment for the test.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>