bash completion: Add completion for 'git ls-files'

bash completion: Add completion for 'git ls-files'

Add completions for all long options specified in the docs
    --cached --deleted --modified --others --ignored
    --stage --directory --no-empty-directory --unmerged
    --killed --exclude= --exclude-from=
    --exclude-per-directory= --exclude-standard
    --error-unmatch --with-tree= --full-name
    --abbrev --ignored --exclude-per-directory

Signed-off-by: Lee Marlow <lee.marlow@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>