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/structs.h b/src/structs.h
index f1bbf17..7e21f0f 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4470,7 +4470,8 @@
     char_u	*pum_kind;	// extra kind text (may be truncated)
     char_u	*pum_extra;	// extra menu text (may be truncated)
     char_u	*pum_info;	// extra info
-    int		 pum_score;	// fuzzy match score
+    int		pum_score;	// fuzzy match score
+    int		pum_idx;	// index of item before sorting by score
 } pumitem_T;
 
 /*