lib/pull: Add `timestamp-check-from-rev`

lib/pull: Add `timestamp-check-from-rev`

The way `timestamp-check` works might be too restrictive in some
situations. Essentially, we need to support the case where users want to
pull an older commit than the current tip, but while still guaranteeing
that it is newer than some even older commit.

This will be used in Fedora CoreOS. For more information see:
https://github.com/coreos/rpm-ostree/pull/2094
https://github.com/coreos/fedora-coreos-tracker/issues/481