Feat: Show some "untracked" marker in the diff component

[lucc]

Requested feature

I would like to have an indicator in my statusline if the current file is untracked by git. (Obviously only if I am in a git repo).

This is what t looks like in airline (the crossed out "E" after the branch name):

I think it would be nice to display it in the diff component for untracked files.

Motivation

I am coming from airline and they have it. I found it to be useful. I think it is a good extension of the diff component because it is essentially unused for untracked files.

I consider the diff component a hint to the user that they have uncommited work in this file. The same is true for untracked files for which the user gets no hint in lualine.