improve hierarchy display of snapshots

[lnussel]

The default list display of snapper does not state the "parent" of a snapshot. So after a rollback it's not clear which snapshots are based on common history. The "pre" column is only used snapshots of type "post"

Example of a normal system:

    # | Type   | Pre # | Date                     | User | Used Space | Cleanup | Description  | Userdata     
------+--------+-------+--------------------------+------+------------+---------+--------------+--------------
   0  | single |       |                          | root |            |         | current      |              
 865* | single |       | Mon Dec 16 13:51:18 2019 | root | 203.57 MiB |         |              |              
 956  | pre    |       | Tue Aug  4 13:07:58 2020 | root | 628.38 MiB | number  | zypp(zypper) | important=yes
 957  | post   |   956 | Tue Aug  4 13:09:24 2020 | root | 326.64 MiB | number  |              | important=yes

On a transactional system the "pre" column is always empty and using space without need:

# snapper list
   # | Type   | Pre # | Date                     | User | Used Space | Cleanup | Description             | Userdata     
-----+--------+-------+--------------------------+------+------------+---------+-------------------------+--------------
  0  | single |       |                          | root |            |         | current                 |              
301  | single |       | Tue May  4 13:06:56 2021 | root |  44.82 MiB | number  | Snapshot Update of #300 | important=yes
302  | single |       | Tue May  4 15:38:48 2021 | root |   6.27 MiB | number  | Snapshot Update of #301 | important=yes
303* | single |       | Tue May  4 15:52:45 2021 | root |  17.75 MiB | number  | Snapshot Update of #302 | important=yes

As a workaround for missing history information the description is abused to contain the parent snapshot.

Maybe the list display could be improved to be able to express the history without the description?
Maybe a different style needs to be used for normal vs transactional systems?

[aschnell]

It is already possible to change the columns using the --columns parameter.