patch 8.1.0685: get_buf_tv() is named inconsistently

Problem:    get_buf_tv() is named inconsistently.
Solution:   Rename it to tv_get_buf(). (Yegappan Lakshmanan, closes #3759)

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    685,
 /**/
     684,
 /**/