create floating window with commit diff when editing git commit message

lua/core/auto.lua

@@ -18,6 +18,57 @@ auto('FileType', { -- start git messages in insert mode
   command = 'startinsert | 1'
 })
 
+auto('FileType', { -- show git cached diff
+  group = 'bufcheck',
+  pattern = { 'gitcommit', 'gitrebase', },
+  callback = function()
+    local function lower(a, b)
+      if a > b then
+        return b
+      else
+        return a
+      end
+    end
+
+    local function tablelength(T)
+      local count = 0
+      for _ in pairs(T) do count = count + 1 end
+      return count
+    end
+
+    local diff = io.popen('git diff --cached', "r")
+    local diffl = {}
+    for i in diff:lines() do
+      table.insert(diffl, i)
+    end
+    diff:close()
+
+    if next(diffl) == nil then
+      vim.notify('No diff to show :(', vim.log.levels.INFO, {
+        title = 'Neovim Config' })
+      return
+    end
+
+    local buf = a.nvim_create_buf(true, false)
+    a.nvim_buf_set_lines(buf, 0, -1, false, diffl)
+    a.nvim_buf_set_name(buf, 'Git Commit Diff')
+    a.nvim_buf_set_option(buf, 'ft', 'diff')
+    a.nvim_buf_set_option(buf, 'readonly', true)
+    a.nvim_buf_set_option(buf, 'modifiable', false)
+    a.nvim_buf_set_option(buf, 'modified', false)
+    local win = a.nvim_open_win(buf, false, {
+      row = 1,
+      col = 80,
+      relative = 'win',
+      width = 80,
+      height = lower(vim.o.lines - vim.o.cmdheight - 4, tablelength(diffl)),
+      border = 'shadow',
+      style = 'minimal',
+    })
+  end,
+})
+
+
 auto('BufRead', { -- return to last place
   pattern = '*',
   command = [[call setpos(".", getpos("'\""))]]