nvim: Fix automatic packer installation

Updated to new recommended bootstrapping for packer itself, straight
from the project respository.

nvim/.config/nvim/lua/plugins.lua

@@ -1,9 +1,15 @@
-local install_path = vim.fn.stdpath("data") .. "/pack/packer/start/packer.nvim"
-
-if vim.fn.empty(vim.fn.glob(install_path)) > 0 then
-    vim.cmd("!git clone https://github.com/wbthomason/packer.nvim " ..
-        install_path)
+local ensure_packer = function()
+    local fn = vim.fn
+    local install_path = fn.stdpath('data') .. '/site/pack/packer/start/packer.nvim'
+    if fn.empty(fn.glob(install_path)) > 0 then
+        fn.system({ 'git', 'clone', '--depth', '1', 'https://github.com/wbthomason/packer.nvim', install_path })
+        vim.cmd [[packadd packer.nvim]]
+        return true
     end
+    return false
+end
+
+local packer_bootstrap = ensure_packer()
 
 -- Compile on plugin edits
 vim.api.nvim_create_autocmd({ "BufWritePost" }, {
@@ -233,4 +239,9 @@ require("packer").startup(function()
     }
     require('plug._cmp')
 
+    -- Automatically set up your configuration after cloning packer.nvim
+    -- Put this at the end after all plugins
+    if packer_bootstrap then
+        require('packer').sync()
+    end
 end)