builtin/checkout.c: complete parallel checkout support

builtin/checkout.c: complete parallel checkout support

Pathspec-limited checkouts (like `git checkout *.txt`) are performed by
a code path that doesn't yet support parallel checkout because it calls
checkout_entry() directly, instead of unpack_trees(). Let's add parallel
checkout support for this code path too.

The transient cache entries allocated in checkout_merged() are now
allocated in a mem_pool which is only discarded after parallel checkout
finishes. This is done because the entries need to be valid when
run_parallel_checkout() is called.

Signed-off-by: Matheus Tavares <redacted>
Signed-off-by: Junio C Hamano <redacted>