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/popupmnu.c b/src/popupmnu.c
index f3a9f39..b0fe043 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -1103,7 +1103,7 @@
 	    return;
 	for (idx = 0, li = list->lv_first; li != NULL; li = li->li_next, ++idx)
 	{
-	    char_u *text = get_tv_string_chk(&li->li_tv);
+	    char_u *text = tv_get_string_chk(&li->li_tv);
 
 	    balloon_array[idx].pum_text = vim_strsave(
 					   text == NULL ? (char_u *)"" : text);