lib/repo-refs: Fix resolving collection-refs

lib/repo-refs: Fix resolving collection-refs

My last commit "lib/repo-refs: Resolve collection-refs in-memory and in
parent repos" changed ostree_repo_resolve_collection_ref() to check the
in-memory set of refs *after* failing to find the ref on disk but that's
not what we want. We want to use the in-memory set of refs first,
because those are the most up to date commits, and then fall back to the
on-disk repo and finally fall back to checking any parent repo. This
commit makes such a change to the order of operations, which is
consistent with how ostree_repo_resolve_rev() works.

Aside from this change being logical, it also fixes some unit test
failures on an unmerged branch of flatpak:
https://github.com/flatpak/flatpak/pull/2705

Also, tweak the comments here.

Closes: #1825
Approved by: jlebon