Git state doesn't update on Linux for all untracked files

[kuychaco]

Since on Linux we can't watch entire directories and we can only watch individual files, workspace-change-observer.js currently watches the index file in the .git directory and any open buffers in Atom. This means that if changes are made to untracked files outside of Atom, the editor's git state will not update.

To address this we can periodically re-fetch all the git state and update.