Remove `refs/refs-internal.h` from `builtin/show-ref.c`
The refs/refs-internal.h
header file exposes internal functionality of the refs library. But this is supposed to be used within the refs/
namespace. Currently builtin/show-ref.c
seems to be using this code, worthwhile cleaning this up.
More context: https://lore.kernel.org/git/20240426211529.GD13703@coredump.intra.peff.net/