fix: show bug when descendant is already fixed

If a commit needs fixing but its descendant does not, we currently
skip rewriting the descendant, which means it will instead be rebased
when the transactions finished. That usually results in
conflicts. This adds a test case for that case.

lib/tests/test_fix.rs

@@ -326,6 +326,50 @@ fn test_unchanged_file_is_fixed() {
     assert_eq!(*new_commit_b.tree_id(), expected_tree_b.id());
 }
 
+/// If a descendant is already correctly formatted, it should still be rewritten
+/// but its tree should be preserved.
+#[test]
+fn test_already_fixed_descendant() {
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    let mut tx = repo.start_transaction();
+    let path1 = RepoPath::from_internal_string("file1");
+    let tree1 = create_tree(repo, &[(path1, "fixme:content")]);
+    let commit_a = create_commit(
+        &mut tx,
+        vec![repo.store().root_commit_id().clone()],
+        tree1.id(),
+    );
+
+    let tree2 = create_tree(repo, &[(path1, "CONTENT")]);
+    let _commit_b = create_commit(&mut tx, vec![commit_a.clone()], tree2.id());
+
+    let root_commits = vec![commit_a.clone()];
+    let file_fixer = TestFileFixer::new();
+
+    let summary = fix_files(
+        root_commits,
+        &EverythingMatcher,
+        true,
+        tx.repo_mut(),
+        &file_fixer,
+    )
+    .unwrap();
+
+    // TODO: Should have fixed both commits
+    assert_eq!(summary.rewrites.len(), 1);
+    assert!(summary.rewrites.contains_key(&commit_a));
+    assert_eq!(summary.num_checked_commits, 2);
+    assert_eq!(summary.num_fixed_commits, 1);
+
+    let new_commit_a = repo
+        .store()
+        .get_commit(summary.rewrites.get(&commit_a).unwrap())
+        .unwrap();
+    assert_eq!(*new_commit_a.tree_id(), tree2.id());
+}
+
 #[test]
 fn test_parallel_fixer_basic() {
     let test_repo = TestRepo::init();