vim-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 vim/vim#10135,
            closes vim/vim#2173)
db0ea7f2b0

Cherry-pick FOR_ALL_BUF_WININFO from patch 8.2.0500.
Cherry-pick test_changelist.vim change from patch 8.2.3795.

src/nvim/buffer_defs.h

@@ -289,6 +289,7 @@ struct wininfo_S {
   winopt_T wi_opt;              // local window options
   bool wi_fold_manual;          // copy of w_fold_manual
   garray_T wi_folds;            // clone of w_folds
+  int wi_changelistidx;         // copy of w_changelistidx
 };
 
 /*