vim-patch:8.1.1466: not updating priority on existing sign

Problem:    Not updating priority on existing sign.
Solution:   Set the sign priority.  Add a test. (Yegappan Lakshmanan)
58a7f87c86

src/nvim/sign.c

@@ -284,6 +284,7 @@ void buf_addsign(
         && sign_in_group(sign, groupname)) {
       // Update an existing sign
       sign->typenr = typenr;
+      sign->priority = prio;
       return;
     } else if (lnum < sign->lnum) {
       insert_sign_by_lnum_prio(buf, prev, id, groupname, prio, lnum, typenr);