t3001: test ls-files -o ignored/dir

t3001: test ls-files -o ignored/dir

When you have "t" directory that is marked as ignored in the top-level
.gitignore file (or $GIT_DIR/info/exclude), running

    $ git ls-files -o --exclude-standard

from the top-level correctly excludes files in "t" directory, but
any of the following:

    $ git ls-files -o --exclude-standard t/
    $ cd t && git ls-files -o --exclude-standard

would show untracked files in that directory.

Signed-off-by: Junio C Hamano <gitster@pobox.com>