git_ui: Add support for generating commit messages with an LLM (#26227)

This PR finishes up the support for generating commit messages using an
LLM.

We're shelling out to `git diff` to get the diff text, as it seemed more
efficient than attempting to reconstruct the diff ourselves from our
internal Git state.


https://github.com/user-attachments/assets/9bcf30a7-7a08-4f49-a753-72a5d954bddd

Release Notes:

- Git Beta: Added support for generating commit messages using a
language model.

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>