worktree: teach worktree to lazy-load "prunable" reason

worktree: teach worktree to lazy-load "prunable" reason

Add worktree_prune_reason() to allow a caller to discover whether a
worktree is prunable and the reason that it is, much like
worktree_lock_reason() indicates whether a worktree is locked and the
reason for the lock. As with worktree_lock_reason(), retrieve the
prunable reason lazily and cache it in the `worktree` structure.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rafael Silva <rafaeloliveira.cs@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>