ls-files: allow relative pathspec

ls-files: allow relative pathspec

git ls-files used to error out if given paths which point outside the current
working directory, such as '../'. We now allow such paths and the output is
analogous to git grep -l.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>