Select working tree or index in diff mode

**Is your feature request related to a problem? Please describe.**
There doesn't seem to be any way to produce diffs using the working tree or index in diff mode, such that enter can then be used to drill into changed file diffs. It seems only actual refs like HEAD can be chosen.

Such diffs are (I think) produced by `git diff some-commit` (working tree) `git diff --cached some-commit` (index).

Although I wonder if I'm missing some obvious option or workaround here since it seems a basic, and essential (for me anyway) feature.

**Describe the solution you'd like**
To be able to select something like `Working Tree` or `Index` in the diff tool instead of a git ref.

**Describe alternatives you've considered**
Committing all changes first and selecting HEAD as the ref.
Doing something clever with Custom Command (not sure if possible).