fix(lua): vim.diff is nil in uv.new_work() thread #34909

Problem:
The "gitsigns" plugin runs `vim.diff` in a thread (`uv.new_work`), but
`vim.diff` is nil in that context:

    Lua callback:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: bad argument #1 to 'decode' (string expected, got nil)
    stack traceback:
      [C]: in function 'decode'
      …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: in function <…/gitsigns.nvim/lua/gitsigns/diff_int.lua:29>
    Luv thread:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:63: attempt to call field 'diff' (a nil value)

Solution:
Revert the `stdlib.c` change (set `vim.diff` instead of `vim._diff`).

runtime/lua/vim/_editor.lua

@@ -1317,9 +1317,6 @@ require('vim._options')
 --- Remove at Nvim 1.0
 ---@deprecated
 vim.loop = vim.uv
---- Renamed to `vim.text.diff`, remove at Nvim 1.0
----@deprecated
-vim.diff = vim._diff ---@type fun(a: string, b: string, opts?: vim.text.diff.Opts): string|integer[][]?
 
 -- Deprecated. Remove at Nvim 2.0
 vim.highlight = vim._defer_deprecated_module('vim.highlight', 'vim.hl')

runtime/lua/vim/_meta/misc.lua

@@ -14,3 +14,11 @@
 --- @param ... any
 --- @return any
 function vim.call(func, ...) end
+
+--- Renamed to `vim.text.diff`, remove at Nvim 1.0
+---@deprecated
+---@param a string First string to compare
+---@param b string Second string to compare
+---@param opts? vim.text.diff.Opts
+---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
+function vim.diff(a, b, opts) end

runtime/lua/vim/text.lua

@@ -73,7 +73,8 @@ local M = {}
 ---@param opts? vim.text.diff.Opts
 ---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
 function M.diff(...)
-  return vim._diff(...)
+  ---@diagnostic disable-next-line: deprecated
+  return vim.diff(...)
 end
 
 local alphabet = '0123456789ABCDEF'