core: Add detached metadata, readd metadata to commits

Previously I thought we'd have to ditch the current commit
format to avoid a{sv} due to

See https://bugzilla.gnome.org/show_bug.cgi?id=673012

But I realized that we don't really have to care about
unpacking/repacking commit objects, so let's just re-expose the
existing metadata a{sv} in commits in the API.

Also, add support for "detached" metadata that can be updated at any
time post-commit.  This is specifically designed for GPG signatures.

https://bugzilla.gnome.org/show_bug.cgi?id=707379

src/libostree/ostree-repo-prune.c

@@ -64,6 +64,13 @@ maybe_prune_loose_object (OtPruneData        *data,
 
           if (info)
             {
+              if (objtype == OSTREE_OBJECT_TYPE_COMMIT)
+                {
+                  gs_unref_object GFile *detached_metadata =
+                    _ostree_repo_get_commit_metadata_loose_path (data->repo, checksum);
+                  if (!ot_gfile_ensure_unlinked (detached_metadata, cancellable, error))
+                    goto out;
+                }
               if (!gs_file_unlink (objf, cancellable, error))
                 goto out;
               data->freed_bytes += g_file_info_get_size (info);