patch 9.1.0484: Sorting of completeopt+=fuzzy is not stable

Problem:  Sorting of completeopt+=fuzzy is not stable.
Solution: Compare original indexes when scores are the same.
          (zeertzjq)

closes: #14988

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/insexpand.c b/src/insexpand.c
index 74be94c..fafa7a2 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -1203,11 +1203,13 @@
  * pumitem qsort compare func
  */
     static int
-ins_compl_fuzzy_sort(const void *a, const void *b)
+ins_compl_fuzzy_cmp(const void *a, const void *b)
 {
     const int sa = (*(pumitem_T *)a).pum_score;
     const int sb = (*(pumitem_T *)b).pum_score;
-    return sa == sb ? 0 : sa < sb ? 1 : -1;
+    const int ia = (*(pumitem_T *)a).pum_idx;
+    const int ib = (*(pumitem_T *)b).pum_idx;
+    return sa == sb ? (ia == ib ? 0 : (ia < ib ? -1 : 1)) : (sa < sb ? 1 : -1);
 }
 
 /*
@@ -1355,8 +1357,13 @@
     } while (compl != NULL && !is_first_match(compl));
 
     if (compl_fuzzy_match && compl_leader != NULL && lead_len > 0)
+    {
+	for (i = 0; i < compl_match_arraysize; i++)
+	    compl_match_array[i].pum_idx = i;
 	// sort by the largest score of fuzzy match
-	qsort(compl_match_array, (size_t)compl_match_arraysize, sizeof(pumitem_T), ins_compl_fuzzy_sort);
+	qsort(compl_match_array, (size_t)compl_match_arraysize,
+				       sizeof(pumitem_T), ins_compl_fuzzy_cmp);
+    }
 
     if (!shown_match_ok)    // no displayed match at all
 	cur = -1;