Extend gitsigns.diffthis for diff between index and HEAD

[dasupradyumna]

Hello Lewis!
Thank you for a such a simple yet convenient plugin!

Is your feature request related to a problem? Please describe.

I often use gitsigns.diffthis to compare current working tree to the index, and in my workflow, I usually stage my changes / hunks incrementally (parts of a feature, for example) as I work on a feature, before committing. As such, I use git diff --cached HEAD <file> for this purpose (I got used to this feature while using VS Code long back).

One thing I had checked was -

• Open the index for a file using show function.
• Tried to execute diffthis HEAD on the index file, but there was no change / no diff was opened.

Describe the solution you'd like

I am assuming there is a check which disallows diffthis to work on index buffers, so I was wondering if it is possible to modify diffthis function to work when the active buffer is the git index of a file.

Please let me know if this feature already is doable with the current plugin (probably something I missed), otherwise I do think this is a valuable addition to an already amazing plugin.

Additional context

I am hoping for something like this ; except the right side pane should be the git index of the file and not the file from the working tree.

[dasupradyumna]

Hey @lewis6991,
I hope you're not busy. May I get a response? :)

[lewis6991]

This isn't possible at the moment but I plan to add diffthis support for any Gitsigns buffer.

[dasupradyumna]

So after you add that support, I can open the index for a file using show and run diffthis HEAD on the index buffer to get a git-diff between index and HEAD. Did I understand you correctly?