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.

---------

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

crates/git_ui/src/commit_message_prompt.txt

@@ -1,4 +1,8 @@
-You are an expert at writing Git commits. Your job is to write a clear commit message that summarizes the changes.
+You are an expert at writing Git commits. Your job is to write a short clear commit message that summarizes the changes.
+
+If you can accurately express the change in just the subject line, don't include anything in the message body. Only use the body when it is providing *useful* information.
+
+Don't repeat information from the subject line in the message body.
 
 Only return the commit message in your response. Do not include any additional meta-commentary about the task.
 
@@ -10,6 +14,6 @@ Follow good Git style:
 - Do not end the subject line with any punctuation
 - Use the imperative mood in the subject line
 - Wrap the body at 72 characters
-- Use the body to explain *what* and *why* vs. *how*
+- Keep the body short and concise (omit it entirely if not useful)
 
 Here are the changes in this commit: