Add `--git=auto` feature

[ariasuni]

It would be cool to have an option to only enable --git when a git repository is detected. Not sure how practical it is to implement, though.

[sambonbonne]

Isn't this the default when using --git? When I run exa -l --git on a non-git directory, it works like a charm and it doesn't print any Git information (no error, status code is 0 so all is fine).

[Freed-Wu]

How about --icons=(auto|always|never)? Like --git=(auto|always|never).

[ariasuni]

Git is only enabled when a Git repository is found, so this is already implemented.