builtin-commit: Don't parse the parent's GVariant by hand

Instead, use OstreeRepoFile as a handle for the parent commit.
We need to add an accessor for the metadata checksum, as that
hasn't been exposed before.

https://bugzilla.gnome.org/show_bug.cgi?id=707727

src/libostree/ostree-repo-file.c

@@ -362,6 +362,12 @@ ostree_repo_file_tree_get_contents_checksum (OstreeRepoFile  *self)
   return self->tree_contents_checksum;
 }
 
+const char *
+ostree_repo_file_tree_get_metadata_checksum (OstreeRepoFile  *self)
+{
+  return self->tree_metadata_checksum;
+}
+
 /**
  * ostree_repo_file_nontree_get_local:
  *

src/libostree/ostree-repo-file.h

@@ -66,6 +66,7 @@ void ostree_repo_file_tree_set_metadata (OstreeRepoFile *self,
                                           GVariant       *metadata);
 
 const char *ostree_repo_file_tree_get_contents_checksum (OstreeRepoFile  *self);
+const char *ostree_repo_file_tree_get_metadata_checksum (OstreeRepoFile  *self);
 
 gboolean ostree_repo_file_is_tree (OstreeRepoFile  *self);