patch 8.2.4731: the changelist index is not remembered per buffer

Problem:    The changelist index is not remembered per buffer.
Solution:   Keep the changelist index per window and buffer. (closes #10135,
            closes #2173)
diff --git a/src/structs.h b/src/structs.h
index 5ad606e..670e0d2 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -343,6 +343,7 @@
     int		wi_fold_manual;	// copy of w_fold_manual
     garray_T	wi_folds;	// clone of w_folds
 #endif
+    int		wi_changelistidx; // copy of w_changelistidx
 };
 
 /*