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`).
