title: git grep by file extensions
body: <p>I know that, if I wanted to grep for a pattern only on files with certain extensions, I could do this:</p>
  
  <pre><code>// searches recursively and matches case insensitively in only javascript files
  // for "res" from the current directory
  grep -iIr --include=*.js res ./
  </code></pre>
  
  <p>I've been trying to search for a way to do this via git grep as well (to take advantage of git grep's speed from the indexes it stores with its tree), but to no avail. I saw <a href="http://git.661346.n2.nabble.com/git-grep-while-excluding-files-in-a-blacklist-td7195648.html">here</a> that excluding certain file types is not possible.</p>