patch 8.1.0679: sign functions do not take buffer argument as documented

Problem:    Sign functions do not take buffer argument as documented.
Solution:   Use get_buf_tv(). (Yegappan Lakshmanan, closes #3755)
diff --git a/src/version.c b/src/version.c
index fbd6110..2571a8c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    679,
+/**/
     678,
 /**/
     677,