vim-patch:8.0.0672: synconcealed() changes too often #7887

Problem:    Third item of synconcealed() changes too often. (Dominique Pelle)
Solution:   Reset the sequence number at the start of each line.
cc0750dc6e

closes #7589

src/nvim/syntax.c

@@ -881,7 +881,8 @@ static void syn_start_line(void)
   }
 
   next_match_idx = -1;
-  ++current_line_id;
+  current_line_id++;
+  next_seqnr = 1;
 }
 
 /*
@@ -1599,6 +1600,7 @@ get_syntax_attr (
     current_id = 0;
     current_trans_id = 0;
     current_flags = 0;
+    current_seqnr = 0;
     return 0;
   }
 
@@ -2042,6 +2044,7 @@ syn_current_attr (
   current_id = 0;
   current_trans_id = 0;
   current_flags = 0;
+  current_seqnr = 0;
   if (cur_si != NULL) {
     for (int idx = current_state.ga_len - 1; idx >= 0; --idx) {
       sip = &CUR_STATE(idx);