title: git does not display warning overwriting files when doing checkout
body: <p>Let's say we have the following scenario:
  - I created a new branch and checked it out (git checkout -b ).
  - On the new branch I changed a file
  - If I want to switch back to master, git throws the following warning: "error: Your local changes to the following files would be overwritten by checkout..."</p>
  
  <p>I think this is how git should behave. I have other colleagues who work on the same project but when they do the same as above, the warning does not appear and git simply switches the branch. It even seems to merge the files automatically and git status shows the same on both branches. Why do I get the warning and they don't? Is this a setting thing?</p>
  
  <p>Thanks in advance.</p>