Add "undo last commit"

### Godot version

v4.6.1.stable.official [14d19694e]

### Godot Git Plugin version

3.2.1

### System information

Fedora Linux 43

### Problem statement

Having an "undo last commit" command (instead of having to switch to a terminal) would be very helpful.

### Proposed solution

Implement an "Undo Last Commit" menu item (maybe below "Force Push"?) that will execute `git reset --soft HEAD~1`.