title: vim wildmode=longest,list doesn't work when files are on different directories
body: <p>Vim file name autocompletion doesn't work if files are on different directory levels. Steps to reproduce:</p>
  
  <ol>
  <li><code>set wildmode=longest,list</code> in vimrc</li>
  <li><code>vim usa1.txt test/usa2.txt</code></li>
  <li>type <code>:b u</code> in EX mode, then press <kbd>TAB</kbd>, it is expected to auto complete the word up to <code>:b usa</code>, which is the common header of two file names. But it just doesn't work.</li>
  </ol>
  
  <p>Any solutions? Thanks a lot!</p>
  
  <p>PS: if the above two files are placed on the SAME directory, it works!!!</p>