ls-files: honour per-directory ignore file from higher directories.

ls-files: honour per-directory ignore file from higher directories.

When git-ls-files -o --exclude-per-directory=.gitignore is run
from a subdirectory, it did not read from .gitignore from its
parent directory.  Reading from them makes output from these two
commands consistent:

    $ git ls-files -o --exclude-per-directory=.gitignore Documentation
    $ cd Documentation &&
      git ls-files -o --exclude-per-directory=.gitignore

Signed-off-by: Junio C Hamano <junkio@cox.net>