bufferline: take 'acd' into account
bufferline: take 'acd' into account

vim-airline does use a different section (path/file) depending on
whether 'acd' is set. Later in the bufferline extesion however, it
unconditionally overwrites the 'file' section, regardless of whether
this section is actually used.

Therefore the bufferline section needs to check this option as well.

fixes #1487

tschundler/dotfiles,bling/vim-airline,tschundler/dotfiles,tschundler/dotfiles,yzhou61/vim-airline,rchicoli/vim-airline,chrisbra/vim-airline,unphased/vim-airline