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/ex_cmds.c b/src/ex_cmds.c
index 5cb326e..06df5af 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -8913,7 +8913,7 @@
 	for (li = l->lv_first; li != NULL && !got_int; li = li->li_next)
 	{
 	    ++nr;
-	    fname = get_tv_string(&li->li_tv);
+	    fname = tv_get_string(&li->li_tv);
 	    if (!message_filtered(fname))
 	    {
 		msg_outnum((long)nr);