use old 'tbl_flatten()' impl from neovim

Unlike the `iter(...):flatten()` alternative, this handles lists with
holes.

lua/diffview/utils.lua

@@ -1344,11 +1344,31 @@ function M.merge_sort(t, comparator)
   split_merge(t, 1, #t, comparator)
 end
 
-M.islist = vim.fn.has("nvim-0.10") == 1 and vim.islist or vim.tbl_islist ---@diagnostic disable-line: deprecated
+--- @diagnostic disable-next-line: deprecated
+M.islist = vim.fn.has("nvim-0.10") == 1 and vim.islist or vim.tbl_islist
 
-M.flatten = vim.fn.has("nvim-0.10") == 1 and function(...)
-  return vim.iter(...):flatten(math.huge):totable() 
-end or vim.tbl_flatten ---@diagnostic disable-line: deprecated
+--- @param t table
+--- @return any[]
+function M.flatten(t)
+  local result = {}
+
+  --- @param _t table<any,any>
+  local function recurse(_t)
+    local n = #_t
+    for i = 1, n do
+      local v = _t[i]
+      if type(v) == 'table' then
+        recurse(v)
+      elseif v then
+        table.insert(result, v)
+      end
+    end
+  end
+
+  recurse(t)
+
+  return result
+end
 
 M.path_sep = path_sep