btrfs subvolumes not displayed.

I have a btrfs filesystem with subvolumes. dysk shows only one of the three subvolumes.
```
┌────────────────┬─────┬────┬─────┬─────────┬─────┬──────┬───────────┐
│ filesystem │type │disk│used │ use │free │ size │mount point│
├────────────────┼─────┼────┼─────┼─────────┼─────┼──────┼───────────┤
│/dev/nvme0n1p1 │ xfs │SSD │2.3Gi│29% █▌ │5.6Gi│ 7.9Gi│/ │
│/dev/nvme2n1 │btrfs│SSD │547Mi│18% ▉ │2.5Gi│ 3.0Gi│/tmp │
│/dev/nvme0n1p128│vfat │SSD │1.3Mi│13% ▋ │8.7Mi│10.0Mi│/boot/efi │
└────────────────┴─────┴────┴─────┴─────────┴─────┴──────┴───────────┘
```
When I run `dysk -a` I see the subvolumes but then I see all the other less relevant mounts as well.
```
│/dev/nvme2n1 │ btrfs │SSD │547Mi│18% ▉ │2.5Gi│ 3.0Gi│/var/log/audit │
│/dev/nvme2n1 │ btrfs │SSD │547Mi│18% ▉ │2.5Gi│ 3.0Gi│/var/tmp │
│/dev/nvme2n1 │ btrfs │SSD │547Mi│18% ▉ │2.5Gi│ 3.0Gi│/var/log │
│/dev/nvme2n1 │ btrfs │SSD │547Mi│18% ▉ │2.5Gi│ 3.0Gi│/tmp
```
lsblk shows all three: (var/tmp and /tmp is a bind mount)
```
nvme2n1 259:2 0 3G 0 disk /var/log/audit
/var/tmp
/var/log
/tmp
nvme3n1 259:6 0 3G 0 disk
```
version:
```
dysk 2.9.0
```
Can the default behaviour for dysk not always show all the btrfs subvolumes?

Reproduce the report on a Linux system with btrfs subvolumes and bind mounts, comparing the default output with `dysk -a` on version 2.9.0. Trace how mounted filesystems are selected for the default display; done means the relevant btrfs subvolumes appear without also showing every less relevant mount, with behavior covered by an appropriate test if the repository provides one.

Written by the indexing model from the issue text.