Optionally watch filesystem events to update git status

[OleJoik]

Added a config flag to enable updating git status on file system changes, by default turned off

watch_for_changes = false, -- enable to update status on file system changes

Appears to play well with oils similar config flag.

[refractalize]

I like it, but it doesn't seem to pick up changes to the git status alone. For e.g. if I turn on watch_for_changes in both oil and oil-git-status, then:

1. touch asdf - then the new file is automatically detected by oil, correct.
2. git add asdf - the file's status should be picked up, but it's not detected, not correct.
3. touch asdf2 - the new file is detected AND the asdf file's status is now detected, correct but late.

It's as though we need to watch for changes in the .git directory, not the working copy. Could that be addressed?