title: Repo sync fails to initialize CM13 work tree
body: <p>whenever I try to sync CM13 sources, I get this error:</p>
  
  <pre><code>panos@ubuntu:~/android/cm13$ repo sync -l
  Syncing work tree:  67% (344/512)  fatal: This operation must be run in a work tree
  Traceback (most recent call last):
    File "/home/panos/android/cm13/.repo/repo/main.py", line 531, in &lt;module&gt;
      _Main(sys.argv[1:])
    File "/home/panos/android/cm13/.repo/repo/main.py", line 507, in _Main
      result = repo._Run(argv) or 0
    File "/home/panos/android/cm13/.repo/repo/main.py", line 180, in _Run
      result = cmd.Execute(copts, cargs)
    File "/home/panos/android/cm13/.repo/repo/subcmds/sync.py", line 769, in Execute
      project.Sync_LocalHalf(syncbuf, force_sync=opt.force_sync)
    File "/home/panos/android/cm13/.repo/repo/project.py", line 1247, in Sync_LocalHalf
      self._InitWorkTree(force_sync=force_sync)
    File "/home/panos/android/cm13/.repo/repo/project.py", line 2393, in _InitWorkTree
      raise GitError("cannot initialize work tree")
  error.GitError: cannot initialize work tree
  </code></pre>
  
  <p>...and here's "repo --trace sync -l"</p>
  
  <pre><code>: export GIT_DIR=/home/panos/android/cm13/.repo/projects/hardware/qcom/audio-caf/msm8996.git
  : git rev-parse --verify refs/remotes/github/cm-13.0-caf-8996^0 1&gt;| 2&gt;|
  
  : cd /home/panos/android/cm13/hardware/qcom/audio-caf/msm8996
  : git read-tree --reset -u -v HEAD 1&gt;| 2&gt;|
  fatal: This operation must be run in a work tree
  Traceback (most recent call last):
    File "/home/panos/android/cm13/.repo/repo/main.py", line 531, in &lt;module&gt;
      _Main(sys.argv[1:])
    File "/home/panos/android/cm13/.repo/repo/main.py", line 507, in _Main
      result = repo._Run(argv) or 0
    File "/home/panos/android/cm13/.repo/repo/main.py", line 180, in _Run
      result = cmd.Execute(copts, cargs)
    File "/home/panos/android/cm13/.repo/repo/subcmds/sync.py", line 769, in Execute
      project.Sync_LocalHalf(syncbuf, force_sync=opt.force_sync)
    File "/home/panos/android/cm13/.repo/repo/project.py", line 1247, in Sync_LocalHalf
      self._InitWorkTree(force_sync=force_sync)
    File "/home/panos/android/cm13/.repo/repo/project.py", line 2393, in _InitWorkTree
      raise GitError("cannot initialize work tree")
  error.GitError: cannot initialize work tree
  </code></pre>
  
  <p>I have already retried "repo sync" (what you see is "repo sync -l" which only syncs the work tree locally)</p>
  
  <p>Any help is greatly appreciated. Thanks in advance :D</p>