patch 8.0.0974: resetting a string option does not trigger OptionSet

Problem:    Resetting a string option does not trigger OptionSet. (Rick Howe)
Solution:   Set the origval.
diff --git a/src/version.c b/src/version.c
index 9316815..d5df161 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    974,
+/**/
     973,
 /**/
     972,