dir: Sometimes avoid summary use during local pulls
dir: Sometimes avoid summary use during local pulls

This commit changes flatpak_dir_pull_untrusted_local() to avoid using
the summary file in the source repo if a collection ID is set and use
ostree_repo_resolve_rev() instead in that case. Since summary signatures
are not part of the security model when collection IDs are used, this
doesn't change any security properties. But it does make pulls succeed
when there's nothing to do (i.e. the child repo has no refs because the
parent repo already has the desired commit). In that case, the summary
file will be empty if collection IDs are being used because of
child_repo_ensure_summary().

The end result of this is that "flatpak update --appstream" works when
there's a collection ID set on the remote and the appstream is already
up to date locally. This problem only recently became visible because
flatpak_dir_check_for_appstream_update() was removed in a recent commit.

This is a partial fix for https://github.com/flatpak/flatpak/issues/1615

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak