Don't copy summary for collection-ref mirror subset pulls

Don't copy summary for collection-ref mirror subset pulls

When we're only pulling a subset of the refs available in the remote, it
doesn't make sense to copy the remote's summary (which may not be valid
for the local repo). This makes the check here match the one done
several lines above when we decide whether to error out if there's no
remote summary available.

This extends the fix in https://github.com/ostreedev/ostree/pull/935 for
the case of collection-refs.

Also, add a unit test for this issue, based on the existing one in
pull-test.sh.