fsck: document and test commented & empty line skipList input

fsck: document and test commented & empty line skipList input

There is currently no comment syntax for the fsck.skipList, this isn't
really by design, and it would be nice to have support for comments.

Document that this doesn't work, and test for how this errors
out. These tests reveal a current bug, if there's invalid input the
output will emit some of the next line, and then go into uninitialized
memory. This is fixed in a subsequent change.

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