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/src/git.rs

@@ -49,7 +49,8 @@ actions!(
         Pull,
         Fetch,
         Commit,
-        ExpandCommitEditor
+        ExpandCommitEditor,
+        GenerateCommitMessage
     ]
 );
 action_with_deprecated_aliases!(git, RestoreFile, ["editor::RevertFile"]);

crates/git/src/repository.rs

@@ -217,6 +217,14 @@ pub trait GitRepository: Send + Sync {
 
     /// returns a list of remote branches that contain HEAD
     fn check_for_pushed_commit(&self) -> Result<Vec<SharedString>>;
+
+    /// Run git diff
+    fn diff(&self, diff: DiffType) -> Result<String>;
+}
+
+pub enum DiffType {
+    HeadToIndex,
+    HeadToWorktree,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Deserialize, JsonSchema)]
@@ -577,6 +585,28 @@ impl GitRepository for RealGitRepository {
         )
     }
 
+    fn diff(&self, diff: DiffType) -> Result<String> {
+        let working_directory = self.working_directory()?;
+        let args = match diff {
+            DiffType::HeadToIndex => Some("--staged"),
+            DiffType::HeadToWorktree => None,
+        };
+
+        let output = new_std_command(&self.git_binary_path)
+            .current_dir(&working_directory)
+            .args(["diff"])
+            .args(args)
+            .output()?;
+
+        if !output.status.success() {
+            return Err(anyhow!(
+                "Failed to run git diff:\n{}",
+                String::from_utf8_lossy(&output.stderr)
+            ));
+        }
+        Ok(String::from_utf8_lossy(&output.stdout).to_string())
+    }
+
     fn stage_paths(&self, paths: &[RepoPath]) -> Result<()> {
         let working_directory = self.working_directory()?;
 
@@ -1048,6 +1078,10 @@ impl GitRepository for FakeGitRepository {
     fn check_for_pushed_commit(&self) -> Result<Vec<SharedString>> {
         unimplemented!()
     }
+
+    fn diff(&self, _diff: DiffType) -> Result<String> {
+        unimplemented!()
+    }
 }
 
 fn check_path_to_repo_path_errors(relative_file_path: &Path) -> Result<()> {