title: git diff of an untracked file against branch that has the same file showing diff
body: <p>I have 2 files, file1 and file2.<br>
  file1 is in master.<br>
  branch1 is a branch created off master.<br>
  file2 is in branch1 only.<br></p>
  
  <p>When I do this, I see a diff.</p>
  
  <pre><code>$ git checkout branch1
  Switched to branch 'branch1'
  
  $ cat file2
  branch1 file
  
  $ cp file2 file2_bak
  
  $ git checkout master
  Switched to branch 'master'
  
  $ mv file2_bak file2
  
  $ git diff branch1
  diff --git a/file2 b/file2
  deleted file mode 100644
  index 6fbd376..0000000
  --- a/file2
  +++ /dev/null
  @@ -1 +0,0 @@
  -branch1 file
  </code></pre>
  
  <p>Why is it showing a diff when file2 exists in both branch1 and local?  Is there a way to prevent this diff from showing up?</p>