fsck: warn about '.' and '..' in trees

fsck: warn about '.' and '..' in trees

A tree with meta-paths like '.' or '..' does not work well
with git; the index will refuse to load it or check it out
to the filesystem (and even if we did not have that safety,
it would look like we were overwriting an untracked
directory). For the same reason, it is difficult to create
such a tree with regular git.

Let's warn about these dubious entries during fsck, just in
case somebody has created a bogus tree (and this also lets
us prevent them from propagating when transfer.fsckObjects
is set).

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