Exclude checkpoint archives

[varac]

Using --last 1 will eventually show a checkpoint archive, which I not consider a successful archive (which you documented already in the README).

I use this to show the last successful backup timestamp in the repo:

borg list --sort timestamp  --format '{time}{TAB}{name}{NEWLINE}'  $REPOSITORY  | grep -v '\.checkpoint$' | tail -1 |  cut -f 1

[bebehei]

@varac I'd be happy to receive a PR.