Change git status detecting method
Now I use 'git status -s {path}' command checking a path's status. No longer cached the current directory's git status. It maybe take the view rendering being a little slower, but in this way, I get less code and less independence.
Merge branch 'master' of https://github.com/Xuyuanp/nerdtree

* 'master' of https://github.com/Xuyuanp/nerdtree:
  Update README.markdown