title: commited git folder disappear after checkout
body: <p>I tried to create a new folder under a local git repository like the following.</p>
  
  <ol>
  <li>create a test file name /test/test.txt , under branch "test0"</li>
  <li>git status shows it is untracked.</li>
  <li>git add /test/test.txt, </li>
  <li>git status shows it is Changes to be committed.</li>
  <li>git commit -m "test"</li>
  <li>now I try to create a new branch, using git branch test1</li>
  <li>git checkout test1, now I can not see this test.txt file and the folder.</li>
  <li>git checkout test0, go back to origi branch, but the folder and file test.txt also disappeared (not able to be seen by ls command)</li>
  </ol>
  
  <p>ps. if I use git rm , I could delete this file. I can use git reset HEAD , then git checkout test.txt, then put this file back.</p>
  
  <p>Do you know which step I did wrong? Thanks.</p>