repo: Further optimize ostree_repo_list_objects_set()

In a prior change we discovered that for bad historical reasons
libostree was returning a mapping "object type+checksum" => "metadata"
but the "metadata" was redundant and pointless.

Optimize the prune API to use a (currently internal) object listing
API which returns a set, not a map.  This allows `GHashTable` to
avoid allocating a separate array for the values, neatly cutting
memory usage in half (from ~13MB to ~6MB) on my test case of a
dry-run prune of a FCOS build.

src/libostree/ostree-repo-private.h

@@ -513,6 +513,12 @@ _ostree_repo_verify_bindings (const char  *collection_id,
                               GVariant    *commit,
                               GError     **error);
 
+GHashTable *
+ostree_repo_list_objects_set (OstreeRepo                  *self,
+                              OstreeRepoListObjectsFlags   flags,
+                              GCancellable                *cancellable,
+                              GError                     **error);
+
 /**
  * OstreeRepoAutoTransaction:
  *