nvim: after/plugin/statusline: Add a git branch ahead behind component

Taken from
https://www.reddit.com/r/neovim/comments/t48x5i/git_branch_aheadbehind_info_status_line_component

nvim/.config/nvim/after/plugin/statusline.lua

@@ -2,6 +2,35 @@ local fn  = vim.fn
 local api = vim.api
 local cmd = vim.cmd
 
+local gstatus = { ahead = 0, behind = 0 }
+local update_gstatus = function()
+  local Job = require 'plenary.job'
+  Job:new({
+    command = 'git',
+    args    = { 'rev-list', '--left-right', '--count', 'HEAD...@{upstream}' },
+    on_exit = function(exit_job, _)
+      local res = exit_job:result()[1]
+      if type(res) ~= 'string' then
+        gstatus = { ahead = 0, behind = 0 };
+        return
+      end
+      local ok, ahead, behind = pcall(string.match, res, "(%d+)%s*(%d+)")
+      if not ok then
+        ahead, behind = 0, 0
+      end
+      gstatus = { ahead = ahead, behind = behind }
+    end,
+  }):start()
+end
+
+if _G.Gstatus_timer == nil then
+  _G.Gstatus_timer = vim.loop.new_timer()
+else
+  _G.Gstatus_timer:stop()
+end
+
+_G.Gstatus_timer:start(0, 2000, vim.schedule_wrap(update_gstatus))
+
 local M = {}
 
 local colors = {
@@ -29,6 +58,7 @@ highlight("Filename"    , colors.blue   , colors.bg)
 highlight("Fileformat"  , colors.purple , colors.bg)
 
 highlight("GitBranch"   , colors.orange , colors.bg)
+highlight("GitStatus"   , colors.magenta, colors.bg)
 highlight("DiffAdded"   , colors.green  , colors.bg)
 highlight("DiffChanged" , colors.orange , colors.bg)
 highlight("DiffRemoved" , colors.red    , colors.bg)
@@ -166,10 +196,19 @@ M.progress = function()
   return string.format(" %s%s", "%#Progress#", "%3p%% ")
 end
 
+M.git_ahead_behind_status = function()
+  return
+    vim.b.gitsigns_status_dict
+    and
+    string.format(" %s%s%d%s%d", '%#GitStatus#', ' ', gstatus.behind, ' ', gstatus.ahead)
+    or ""
+end
+
 M.set_active = function(self)
   return table.concat {
     self:get_filename(),
     self:get_git_status(),
+    self:git_ahead_behind_status(),
     "%=",
     self:diagnostic_status(),
     self:get_line_col(),