b:gitsigns_head is not updated after checkout -b from Neovim's command line

[echasnovski]

Description

When branch is checkout with checkout -b from Neovim's command line, b:gitsigns_head does not display the current branch. If checkout is done with checkout, everything works as expected.

Neovim version

NVIM v0.5.0

Operating system and version

Ubuntu 18.04.2

Steps to reproduce

1. Create git repository and make initial commit.
2. Create any new file (say, 'file.md').
3. nvim -nu minimal.lua file.md
4. Check current value with :echo b:gitsigns_head. This should have the name of default branch.
5. Run :!git checkout -b feature. This should change branch to feature. Check it with !git branch.
6. Check again the current value with :echo g:gitsigns_head. This stayed the same.

NOTE: if instead of one command :!git checkout -b feature run two consecutive :!git branch feature and :!git checkout feature, everything works as expected (b:gitsigns_head has value feature).

Expected behavior

Value of b:gistigns_head always shows the name of current HEAD.

Actual behavior

Value of b:gistigns_head not always has the name of current HEAD, as it can have the value HEAD before checkout -b.

Gitsigns debug messages

No response

Minimal config

vim.cmd [[set runtimepath=$VIMRUNTIME]]
vim.cmd [[set packpath=/tmp/nvim/site]]
local package_root = '/tmp/nvim/site/pack'
local install_path = package_root .. '/packer/start/packer.nvim'

if vim.fn.isdirectory(install_path) == 0 then
  print("Installing Gitsigns and dependencies.")
  vim.fn.system { 'git', 'clone', '--depth=1', 'https://github.com/wbthomason/packer.nvim', install_path }
end

-- Load plugins
require('packer').startup {
  {
    'wbthomason/packer.nvim',
    {
      'lewis6991/gitsigns.nvim',
      requires = { 'nvim-lua/plenary.nvim' },
    },
    -- ADD PLUGINS THAT ARE _NECESSARY_ FOR REPRODUCING THE ISSUE
  },
  config = {
    package_root = package_root,
    compile_path = install_path .. '/plugin/packer_compiled.lua',
    display = { non_interactive = true },
  },
}

_G.load_config = function()
  require('gitsigns').setup{
    debug_mode = true, -- You must add this to enable debug messages
    -- GITSIGNS CONFIG THAT ARE _NECESSARY_ FOR REPRODUCING THE ISSUE
  }

  -- ADD INIT.LUA SETTINGS THAT ARE _NECESSARY_ FOR REPRODUCING THE ISSUE

  -- ALSO INCLUDE GITSIGNS DEBUG MESSAGES
  vim.cmd('Gitsigns debug_messages')
end

require('packer').sync()

vim.cmd [[autocmd User PackerComplete ++once echo "Ready!" | lua load_config()]]

[lewis6991]

This happens because git checkout -b doesn't modify .git/index.

Solution is to watch a different file.