fix calculation of removed lines for hunk summary.

apologies for the spam.  i jumped the gun too fast.  the calculation of deleted lines was off (since there's only ever going to be 1 sign per deleted section of code).  i resurrected my [original implementation](https://github.com/bling/vim-airline/blob/16f2d7013d48be4456c6fdf882138bdadca9b908/autoload/airline/extensions/hunks.vim).

there's a bit of duplicated code now, since the `is_added` and friends is getting called here and in `process_hunks`.  let me know if you want to keep it as is, or if it should be moved.

cheers!