Add --forget-last-snapshot option for "forget"

[torfason]

I expanded the script and added a post with a working version in the Recipes category:

Script to forget old snaphots relative to current time (written in R)

The restic forget command operates relative to the newest snapshot in each group. In some cases, it is important do remove snapshots relative to the current time instead. It is formatted as a reprex, with example output included as comments. The script can also be modified to implement forgetting according to other custom rules that may not be supported by the built-in command. WARNING: This script can result in the forgetting of all backups in a group and should be used with care. In most cas…