BUG: update-grub silently fails if root filesystem is inside unsupported by GRUB2 volume

[S-trace]

Hello.

I have a following partitions layout:
sda1 - esp
sda2 - LUKS
Inside LUKS I have LVM
Inside LVM I have regular volume "BOOT" and lvm-thin pool "THIN"
Inside THIN lvm-thin pool I have "ROOT" volume containing BTRFS.

If I run update-grub - snapshots-btrfs GRUB hook fails at root_fs=$(${grub_probe} --target="fs" /) due to set -e at the beginning of snapshots-btrfs GRUB hook without any error messages (which is awful).

If I run grub-probe --target="fs" / manually - I got following error:

grub-probe: error: disk `lvmid/DGpbHM-b8Aa-beeT-SQpo-12lY-d7a3-WGZSGu/uBMaVC-mZVD-eZjJ-GoK9-KY7P-1oRw-4lcagj' not found.

This error occurs because GRUB can't yet handle lvm-thin volumes (and because of this I have separate BOOT volume in LVM).

But the main issue is update-grub silently fails because of snapshots-btrfs GRUB hook.

Fix is simple - do not evaluate grub-probe result into variable, but use it directly in test clause:

[[ $("${grub_probe} --target="fs" /)" != "btrfs" ]] && exit 0

Failure of commands in test clause in Bash does not trigger set -v error and can be handled in script to exit gracefully.