pull: Also skip partial commits for deltas if no summary file

pull: Also skip partial commits for deltas if no summary file

I was playing around in a FAH vagrant box, and hit:

```
Receiving delta parts: 3/4 453.2 kB/s 1.8 MB/145.8 MB
error: opcode set-read-source: No such file object b6e54ba3471b9c116ce6b9bfbf9e55fec60d35cfdb9ae5ae1ee219af02a591b7
```

This is because this host version doesn't yet have
https://github.com/ostreedev/ostree/pull/710
which incidentally fixed this for the case where the OS vendor is using
summary files.

Some organizations may not be using summary files - at least we still try to
support that case. So let's copy the logic very recently added in that commit to
handle the legacy case too.

No new tests since this is a nice-to-have - we really do
expect people to be using summary files now.

Closes: #739
Approved by: jlebon