Replacing brackets.fs.stat() with FileSystem.replace()

Hi @zaggino !
I created a branch for fixing the issue #853 . By using the deprecated `brackets.fs` API, this extension doesn't work on the Intel XDK. 
I used `FileSystem.replace` instead of `brackets.fs.stat` if you don't mind to have one more entry on the index map, since we have only one .git/ folder per project. File watchers shouldn't be a problem, but if you think that this is not a good solution, we can use `fs.exists` from Node APIs.

Thanks