I was also on a Windows machine using Git Shell when I encountered the same error.
However, at the time I had multiple Git terminals open.
The first terminal received the error you posted about above and the other terminal had previously ran the grunt serve terminal command from yeoman (linked below). The second terminal needed to remain open to host a local server instance.
Shutting down all terminal windows running ongoing processes can cause the error to go away.
At least that's what worked for me. After I shut down the second terminal window, I could easily checkout different branches and manipulate files.
Grunt Serve Command - Yeoman.I/O
http://yeoman.io/learning/