Re: [PATCH v7 4/8] worktree: simplify find_shared_symref() memory ownership model

> If we instead hoist ownership of `worktrees` up to execute_commands()
> -- which calls execute_commands_atomic() or
> execute_commands_non_atomic() -- then we can get by with retrieving
> the worktrees just once, and all those noise changes in update() can
> be dropped since it will no longer be responsible for allocating or
> freeing `worktrees`. For instance:
>
>     static void execute_commands(...)
>     {
>         struct worktree **worktrees;
>         ...
>         worktrees = get_worktrees();
>          if (use_atomic)
>             execute_commands_atomic(commands, si, worktrees);
>         else
>             execute_commands_non_atomic(commands, si, worktrees);
>         free_worktrees(worktrees);
>         ...
>     }
>
> and then execute_commands_atomic() and execute_commands_non_atomic()
> can pass `worktrees` along to update().