title: Git mv fails with Fatal: source directory is empty
body: <p>I would like to maintain history of files within a sub-folder of a repo. But I need to rename the top level folder name. When I run 'git mv dirName newDirName
  I get 'Fatal: source directory is empty;' </p>
  
  <p>My source directory has the following structure:</p>
  
  <pre><code>gitRepoDir
  --.git
  --Source 
  --  -DirLevel2 
  --   --DirLevel3 
  --     --DirLevel4 
  --       --DirLevel5 
  +++       --DirNameToRename 
  +++         --sub1dir 
  +++           --File 
  +++         --sub2dir 
  +++           --File 
  +++         --sub3dir 
  +++           --File 
  +++           --File 
  +++         --sub4dir 
  +++           --File
  </code></pre>
  
  <p>Is there a way to rename the top level folder and maintain history of the files in the below subfolders? Or do I need to create the directory structure first then move the files with the git mv command?</p>