vim: add plugin 'mbbill/undotree' and persist undo history

config/nvim/lua/user/options.lua

@@ -15,6 +15,7 @@ o.pastetoggle = '<F20>'         -- toggle paste with P on Moonlander
 o.swapfile    = false           -- don't use swap files
 o.writebackup = true            -- Make a backup before writing a file...
 o.backup      = false           -- ...but don't keep it around.
+o.undofile    = true            -- write undo history
 
 o.shortmess:append 'I'          -- no intro message when starting Vim
 o.shada = {

config/nvim/lua/user/plugins/undotree.lua

@@ -0,0 +1,20 @@
+return {
+  'mbbill/undotree',
+
+  init = function()
+    vim.g.undotree_WindowLayout = 2  -- tree: left, diff: bottom
+    vim.g.undotree_DiffAutoOpen = 0  -- don't open diff by default
+    vim.g.undotree_SetFocusWhenToggle = 1
+
+
+    vim.g.undotree_TreeNodeShape  = ''
+    vim.g.undotree_TreeVertShape  = '│'
+    vim.g.undotree_TreeSplitShape = '╱'
+    vim.g.undotree_TreeReturnShape = '╲'
+  end,
+
+  config = function()
+    vim.keymap.set('n', '<leader>u', '<cmd>UndotreeToggle<cr>')
+  end
+}
+