print snapshot tree: reduce indentation delta per level

print snapshot tree: reduce indentation delta per level

previous:

> `-> foo                         2021-05-28 12:59:36 no-description
>   `-> bar                       2021-06-18 12:44:48 no-description
>     `-> current                                     You are here!

now:

> `-> foo                         2021-05-28 12:59:36 no-description
>  `-> bar                        2021-06-18 12:44:48 no-description
>   `-> current                                       You are here!

So requires less space, allowing deeper snapshot trees to still be
displayed nicely, and looks even better while doing that - the latter
may be subjective though.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>