Pre-emptively import upstream snapshot 5bd4aa9

Pull the latest upstream snapshot, without waiting for the Debian merge request to be landed: https://salsa.debian.org/go-team/packages/debos/merge_requests/2

In particular, pull the fixes below:

• Handle file capabilities by saving xattrs when creating tarballs. (https://phabricator.apertis.org/T5934)
• Fix threading issues when umounting ostree rootfs in non-fakemachine mode. (https://phabricator.apertis.org/T6297)

I've pushed the upstream/1.0.0+git20190906.f5be960 tag already. A new upstream/$SOMETHING should be used to , since we're not really pulling from buster here. Once it is in bullseye we could use upstream/bullseye instead.

The pristine-lfs branch should be fast-forwarded to wip/em/new-upstream-snapshot-pristine-lfs.