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/proto/src/proto.rs

@@ -458,6 +458,8 @@ messages!(
     (GitChangeBranch, Background),
     (CheckForPushedCommits, Background),
     (CheckForPushedCommitsResponse, Background),
+    (GitDiff, Background),
+    (GitDiffResponse, Background),
 );
 
 request_messages!(
@@ -604,6 +606,7 @@ request_messages!(
     (GitCreateBranch, Ack),
     (GitChangeBranch, Ack),
     (CheckForPushedCommits, CheckForPushedCommitsResponse),
+    (GitDiff, GitDiffResponse),
 );
 
 entity_messages!(
@@ -709,6 +712,7 @@ entity_messages!(
     GitChangeBranch,
     GitCreateBranch,
     CheckForPushedCommits,
+    GitDiff,
 );
 
 entity_messages!(