As noticed in https://github.com/flatpak/flatpak/issues/3412 we
regressed at some point and are no longer using from-scratch deltas.
This is caused by an optimization in ostree where it decides to not
use a from-scratch deltas if theres is *some* version of the ref
locally available.
This conflicts with some code in flatpak that pulls *only* the commit
object in order to look for extra data size information so that we can
get the progress reporting right. Unfortunately the existance of
just the object triggers the above causing us to *never* use from-scratch
deltas.
We fix this by throwing away the partial pull in an aborted ostree
transaction.
(cherry picked from commit b371ef9007)
557e809cac