flatpak_dir_find_local_related: Fix handling of subdir extensions

We were not correctly handling the partial refs that ostree_repo_list_refs()
returned, instead assuming they were full refs.

Closes: #1754
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f