fix: buffer data cleared when setting buflisted = false

For posterity: this was a very painful lesson. Turns out "BufDelete"
doesn't mean "buffer was deleted", it means "buffer was deleted from the
buffer list". If you set nobuflisted, BufDelete will be triggered even
though the buffer is still loaded and active.

lua/oil/view.lua

@@ -317,7 +317,7 @@ M.initialize = function(bufnr)
       end, 100)
     end,
   })
-  vim.api.nvim_create_autocmd("BufDelete", {
+  vim.api.nvim_create_autocmd("BufUnload", {
     group = "Oil",
     nested = true,
     once = true,