t1450: refactor ".", "..", and ".git" fsck tests

t1450: refactor ".", "..", and ".git" fsck tests

We check that fsck notices and complains about confusing
paths in trees. However, there are a few shortcomings:

  1. We check only for these paths as file entries, not as
     intermediate paths (so ".git" and not ".git/foo").

  2. We check "." and ".." together, so it is possible that
     we notice only one and not the other.

  3. We repeat a lot of boilerplate.

Let's use some loops to be more thorough in our testing, and
still end up with shorter code.

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