libostree/upgrader: Add an API to retrieve an origin description

This will be used by "rpm-ostree upgrade".

src/libostree/ostree-sysroot-upgrader.c

@@ -329,6 +329,20 @@ ostree_sysroot_upgrader_set_origin (OstreeSysrootUpgrader *self,
   return ret;
 }
 
+/**
+ * ostree_sysroot_upgrader_get_origin_description:
+ * @self: Upgrader
+ *
+ * Returns: A one-line descriptive summary of the origin, or %NULL if unknown
+ */
+char *
+ostree_sysroot_upgrader_get_origin_description (OstreeSysrootUpgrader *self)
+{
+  if (!self->origin)
+    return NULL;
+  return g_key_file_get_string (self->origin, "origin", "refspec", NULL);
+}
+
 /**
  * ostree_sysroot_upgrader_check_timestamps:
  * @repo: Repo