[PATCH] checkout: add --fetch to fetch remote before resolving start-point

> * "--track=inherit,direct" is nonsense and should be rejected
> 
> * currently "--track" has "last one wins" behavior so
>   "--track=inherit --track=direct" behaves like "--track=direct". We
>   should probably keep that so that "--track=fetch --track=direct"
>   behaves like "--track=direct", not "--track=fetch,direct"
> 
> * if "git fetch" fails and the remote tracking ref already exists then
>   we should print a warning and carry on rather than dying which is more
>   convenient if the user or remote server are offline.
> 
> * "git checkout --track=fetch origin/branch" should respect
>   remote.origin.fetch so that we fetch the ref that we're going to
>   checkout. I wonder if we can share this logic with the code that
>   sets the upstream branch.
> 
> * "git checkout --track=fetch origin" should only fetch the remote
>   ref that we're going to checkout, not all the refs from origin. i.e.
>   it should read origin/HEAD to work out what to fetch.