patch 8.0.0513: getting name of cleared highlight group is wrong

Problem:    Getting name of cleared highlight group is wrong. (Matt Wozniski)
Solution:   Only skip over cleared names for completion. (closes #1592)
            Also fix that a cleared group causes duplicate completions.

src/syntax.c

@@ -9949,17 +9949,27 @@ highlight_list_two(int cnt, int attr)
     || defined(FEAT_SIGNS) || defined(PROTO)
 /*
  * Function given to ExpandGeneric() to obtain the list of group names.
- * Also used for synIDattr() function.
  */
     char_u *
 get_highlight_name(expand_T *xp UNUSED, int idx)
+{
+    return get_highlight_name_ext(xp, idx, TRUE);
+}
+
+/*
+ * Obtain a highlight group name.
+ * When "skip_cleared" is TRUE don't return a cleared entry.
+ */
+    char_u *
+get_highlight_name_ext(expand_T *xp UNUSED, int idx, int skip_cleared)
 {
     if (idx < 0)
 	return NULL;
-    /* Items are never removed from the table, skip the ones that were cleared.
-     */
-    while (idx < highlight_ga.ga_len && HL_TABLE()[idx].sg_cleared)
-	++idx;
+
+    /* Items are never removed from the table, skip the ones that were
+     * cleared. */
+    if (skip_cleared && idx < highlight_ga.ga_len && HL_TABLE()[idx].sg_cleared)
+	return (char_u *)"";
 
 #ifdef FEAT_CMDL_COMPL
     if (idx == highlight_ga.ga_len && include_none != 0)