patch 8.1.0615: get_tv function names are not consistent

Problem:    Get_tv function names are not consistent.
Solution:   Rename to tv_get.
diff --git a/src/misc2.c b/src/misc2.c
index a0781d4..3834979 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -6593,7 +6593,7 @@
     *argc = 0;
     for (li = l->lv_first; li != NULL; li = li->li_next)
     {
-	s = get_tv_string_chk(&li->li_tv);
+	s = tv_get_string_chk(&li->li_tv);
 	if (s == NULL)
 	{
 	    int i;