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-private.h

@@ -74,6 +74,10 @@ _ostree_repo_find_object (OstreeRepo           *self,
                           GCancellable         *cancellable,
                           GError             **error);
 
+GFile *
+_ostree_repo_get_commit_metadata_loose_path (OstreeRepo        *self,
+                                             const char        *checksum);
+
 gboolean
 _ostree_repo_has_loose_object (OstreeRepo           *self,
                                const char           *checksum,