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>