Enable fetching in CI. `garden check -D fetch=true` to fetch locally.

"garden my-command -v" should behave the same as "garden -v my-command".
Repeat the variable initialization after the sub-commands have been
parsed to make the behavior match.

Make the "garden <command> -vv" output more compat.

SeqCst is not necessary for handling the exit code atomic.

> While it might seem like the easiest memory ordering to reason about,
> SeqCst ordering is almost never necessary in practice. In nearly all
> cases, regular acquire and release ordering suffice.

https://marabos.nl/atomics/memory-ordering.html#seqcst

Use Release/Acquire pairs to make things easier to understand.

The RefCell that was being used to store it can be made to panic when
running "garden exec" in parallel mode. We only need to be able to clone
the arena by reading from it in a read-only fashion, so use of
UnsafeCell lets us avoid RefCell's limitations and properly implement
the thread-safe clone() for ApplicationContext.