feat: do not close preview when switching dirs (#277)

* feat: do not close preview when cd into dir

* refactor: add helper method to run function after oil buffer loads

* Keep preview window open

* Remove some test logic

* Use `run_after_load` when moving to parent directory

* Remove unnecessary update of current window

* refactor: create helper function for updating preview

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

lua/oil/util.lua

@@ -774,4 +774,26 @@ M.get_visual_range = function()
   return { start_lnum = start_lnum, end_lnum = end_lnum }
 end
 
+---@param bufnr integer
+---@param callback fun()
+M.run_after_load = function(bufnr, callback)
+  if bufnr == 0 then
+    bufnr = vim.api.nvim_get_current_buf()
+  end
+  if vim.b[bufnr].oil_ready then
+    callback()
+  else
+    local autocmd_id
+    autocmd_id = vim.api.nvim_create_autocmd("User", {
+      pattern = "OilEnter",
+      callback = function(args)
+        if args.data.buf == bufnr then
+          callback()
+          vim.api.nvim_del_autocmd(autocmd_id)
+        end
+      end,
+    })
+  end
+end
+
 return M