core: Allow specifying a previous commit tree as a basis for a new commit

It's pretty trivial to map a previously existing commit tree into a
mutable tree too.  While we're here change the command line arguments
for commit so that we can now properly overlay any combination of
directory, commit, or tarfile.

src/libostree/ostree-mutable-tree.c

@@ -28,6 +28,7 @@ struct _OstreeMutableTree
 {
   GObject parent_instance;
 
+  char *contents_checksum;
   char *metadata_checksum;
 
   GHashTable *files;
@@ -43,6 +44,7 @@ ostree_mutable_tree_finalize (GObject *object)
 
   self = OSTREE_MUTABLE_TREE (object);
 
+  g_free (self->contents_checksum);
   g_free (self->metadata_checksum);
 
   g_hash_table_destroy (self->files);
@@ -82,6 +84,20 @@ ostree_mutable_tree_get_metadata_checksum (OstreeMutableTree *self)
   return self->metadata_checksum;
 }
 
+void
+ostree_mutable_tree_set_contents_checksum (OstreeMutableTree *self,
+                                           const char        *checksum)
+{
+  g_free (self->contents_checksum);
+  self->contents_checksum = g_strdup (checksum);
+}
+
+const char *
+ostree_mutable_tree_get_contents_checksum (OstreeMutableTree *self)
+{
+  return self->contents_checksum;
+}
+
 static gboolean
 set_error_noent (GError **error, const char *path)
 {