Enhance git integration and optimize change detection in ServeSession

- Introduced a new function to retrieve the current HEAD commit SHA, allowing for better tracking of changes in the git repository.
- Updated the `git_changed_files` function to accept an initial HEAD parameter, improving the accuracy of change detection.
- Modified `ServeSession` to store the initial HEAD commit SHA at session start, ensuring committed changes are reflected in the auto-selection logic.
- Enhanced tests to verify that committed changes after session start are correctly identified and included in the change tracking.

These changes aim to improve the synchronization process with git and enhance the user experience by ensuring accurate change detection.

tests/tests/git_sync_defaults.rs

@@ -965,3 +965,107 @@ fn stage_ids_stages_new_path_after_rename() {
         "New file (RenamedModule.luau) should be staged after rename + stage_ids"
     );
 }
+
+// ===========================================================================
+// initial_head: committed changes still visible after git commit
+// ===========================================================================
+
+#[test]
+fn committed_changes_still_in_changed_ids_via_initial_head() {
+    let mut session = TestServeSession::new_with_git("git_sync_defaults", |path| {
+        git_commit_all(path, "initial commit");
+    });
+    let info1 = session.wait_to_come_online();
+    let meta1 = info1.git_metadata.unwrap();
+    assert!(meta1.changed_ids.is_empty(), "No changes initially");
+
+    fs::write(
+        session.path().join("src/ModuleA.luau"),
+        "-- committed after session start",
+    )
+    .unwrap();
+    git_commit_all(session.path(), "edit after serve");
+
+    let info2 = session.get_api_rojo().unwrap();
+    let meta2 = info2.git_metadata.unwrap();
+    assert!(
+        !meta2.changed_ids.is_empty(),
+        "Committed changes should still appear in changedIds via initial_head diff"
+    );
+}
+
+#[test]
+fn committed_script_gets_initial_head_hash() {
+    let original_content = "local ModuleA = {}\nreturn ModuleA\n";
+    let modified_content = "-- committed v2\nreturn {}";
+
+    let mut session = TestServeSession::new_with_git("git_sync_defaults", |path| {
+        git_commit_all(path, "initial commit");
+    });
+    session.wait_to_come_online();
+
+    fs::write(session.path().join("src/ModuleA.luau"), modified_content).unwrap();
+    git_commit_all(session.path(), "edit after serve");
+
+    let info2 = session.get_api_rojo().unwrap();
+    let meta2 = info2.git_metadata.unwrap();
+    let read = session.get_api_read(info2.root_instance_id).unwrap();
+
+    if let Some((module_id, _)) = find_instance_by_name(&read.instances, "ModuleA") {
+        assert!(
+            meta2.changed_ids.contains(&module_id),
+            "Committed script should be in changedIds"
+        );
+
+        let hashes = meta2
+            .script_committed_hashes
+            .get(&module_id)
+            .expect("Committed script should have hashes");
+
+        let initial_hash = compute_blob_sha1(original_content);
+        assert!(
+            hashes.contains(&initial_hash),
+            "Hashes should include the initial_head version ({initial_hash}), got {hashes:?}"
+        );
+    }
+}
+
+#[test]
+fn multiple_commits_after_session_start_all_tracked() {
+    let mut session = TestServeSession::new_with_git("git_sync_defaults", |path| {
+        git_commit_all(path, "initial commit");
+    });
+    session.wait_to_come_online();
+
+    fs::write(
+        session.path().join("src/ModuleA.luau"),
+        "-- commit 1 change",
+    )
+    .unwrap();
+    git_commit_all(session.path(), "commit 1");
+
+    fs::write(
+        session.path().join("src/ServerScript.server.luau"),
+        "-- commit 2 change",
+    )
+    .unwrap();
+    git_commit_all(session.path(), "commit 2");
+
+    let info2 = session.get_api_rojo().unwrap();
+    let meta2 = info2.git_metadata.unwrap();
+    let read = session.get_api_read(info2.root_instance_id).unwrap();
+
+    if let Some((module_id, _)) = find_instance_by_name(&read.instances, "ModuleA") {
+        assert!(
+            meta2.changed_ids.contains(&module_id),
+            "ModuleA (committed in first commit) should be in changedIds"
+        );
+    }
+
+    if let Some((server_id, _)) = find_instance_by_name(&read.instances, "ServerScript") {
+        assert!(
+            meta2.changed_ids.contains(&server_id),
+            "ServerScript (committed in second commit) should be in changedIds"
+        );
+    }
+}