How can I place a text file on my homescreen for editing? I would like to place a textfile from my SD-card todo.txt on my home screen so I can click on it to edit the content and when I close it only that file would be saved.

Ideally there would be a git repository behind this file so I can see and eventually rollback any changes I made to that file.
I use "ES File Explorer" as a file explorer on my device. This allows the creation of file shortcuts.

Once installed, just choose the homepage you want the shortcut, press and hold on the homescreen and you should be prompted to add a widget / app / shortcut.

In shortcuts, choose "Es File Explorer" and it then lets you choose the file you want a shortcut for.

Perhaps if used in conjunction with a Git app you can create the shortcut to a file within a repo saved on your device?

Never tried it but don't see why it wouldn't work