patch 8.0.0681: unnamed register only contains the last deleted text

Problem:    Unnamed register only contains the last deleted text when
            appending deleted text to a register. (Wolfgang Jeltsch)
Solution:   Only set y_previous when not using y_append. (Christian Brabandt)
diff --git a/src/version.c b/src/version.c
index 6c2d63c..b06f938 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    681,
+/**/
     680,
 /**/
     679,