title: Ack.vim with ag not using current directory?
body: <p>I am at the moment trying to figure out vim and craft my own .vimrc.</p>
  
  <p>After some research about search-commands and plugins I decided to follow the advice of someone and try out <a href="https://github.com/mileszs/ack.vim" rel="nofollow noreferrer">Ack.vim</a> with <a href="https://github.com/ggreer/the_silver_searcher" rel="nofollow noreferrer">SilverSearcher/Ag</a> for the actual searches. I encountered some problems though, but let me start from the beginning:</p>
  
  <p>When I first installed Ack and tested the "native" Ack via:</p>
  
  <pre><code>:Ack teststring
  </code></pre>
  
  <p>everything worked fine and I got the results in the quickfix-window.</p>
  
  <p>I then proceeded to install SilverSearcher and added the following to my vimrc:</p>
  
  <pre><code>if executable('ag')
    let g:ackprg = 'ag --vimgrep'
  endif
  </code></pre>
  
  <p>...and now when I do the same command as above the search doesn't work anymore. Instead I immediately get an empty quickfix-window and nothing else happens.</p>
  
  <p><strong>However</strong>, if I add a directory to the command, e.g.</p>
  
  <pre><code>:Ack teststring mysubdirectory/
  </code></pre>
  
  <p>the search works.</p>
  
  <p>This confused me. Shouldn't it default to the current directory if none is specified? At least that's how I understand the <a href="https://github.com/ggreer/the_silver_searcher/blob/master/doc/ag.1.md#examples" rel="nofollow noreferrer">documentation</a> and that's how it seems to work for the "native" as well. But as soon as I add the ag-part to my .vimrc it's not anymore.</p>
  
  <p>Can anyone give me a hint at what I am missing here?</p>
  
  <p>PS: In case this is important: I'm on OSX ElCapitan using the terminal-version of VIM8 in iterm2.</p>