Remove trailing white spaces on the changed lines

I am switching from `vim-gitgutter` to this new plugin, it is fast! Thanks.

But I am relying on following function to remove the trailing white spaces on the new added lines.
```
function! GlobalChangedLinesGutter(ex_cmd)
for hunk in GitGutterGetHunks()
for lnum in range(hunk[2], hunk[2]+hunk[3]-1)
let cursor = getcurpos()
silent! execute lnum.a:ex_cmd
call setpos('.', cursor)
endfor
endfor
endfunction

command! -nargs=1 Glines call GlobalChangedLinesGutter()
autocmd BufWritePre * :Glines s/\s\+$//e
```

Is there equivalent API available from `vim-signify` to support this? Thanks.

Start by reading the GlobalChangedLinesGutter function and the BufWritePre autocmd in the issue, then inspect vim-signify's available changed-line APIs. Done means documenting or exposing an equivalent way to apply an Ex command to newly added lines, with the trailing-whitespace example working.

Written by the indexing model from the issue text.