feat: speed up finding changed files in commits (#2645)

We don't need the actual diffs between commits - only the names of the
changed files. This massively improves performance of "release init".

While there *may* be other things we want to do in the future, this
makes enough of a difference that we can probably hold off on larger
changes (at least for the sake of performance).

Fixes #2634

Author: jskeet

internal/gitrepo/gitrepo.go

@@ -477,18 +477,21 @@ func (r *LocalRepository) ChangedFilesInCommit(commitHash string) ([]string, err
 		}
 	}
 
-	patch, err := fromTree.Patch(toTree)
+	changes, err := fromTree.Diff(toTree)
 	if err != nil {
-		return nil, fmt.Errorf("failed to get patch for commit %s: %w", commitHash, err)
+		return nil, fmt.Errorf("failed to get diff for commit %s: %w", commitHash, err)
 	}
 	var files []string
-	for _, filePatch := range patch.FilePatches() {
-		from, to := filePatch.Files()
-		if from != nil {
-			files = append(files, from.Path())
+	for _, change := range changes {
+		from := change.From.Name
+		to := change.To.Name
+		// Deletion or modification
+		if from != "" {
+			files = append(files, from)
 		}
-		if to != nil && (from == nil || from.Path() != to.Path()) {
-			files = append(files, to.Path())
+		// Insertion or rename
+		if to != "" && from != to {
+			files = append(files, to)
 		}
 	}
 	return files, nil

internal/gitrepo/gitrepo_test.go

@@ -1102,6 +1102,16 @@ func TestChangedFilesInCommit(t *testing.T) {
 			commitHash: commitHashes["commit 3"],
 			wantFiles:  []string{"file2.txt"},
 		},
+		{
+			name:       "commit 4",
+			commitHash: commitHashes["commit 4"],
+			wantFiles:  []string{"file2.txt"},
+		},
+		{
+			name:       "commit 5",
+			commitHash: commitHashes["commit 5"],
+			wantFiles:  []string{"file1.txt", "file3.txt"},
+		},
 		{
 			name:       "invalid commit hash",
 			commitHash: "invalid",
@@ -1552,9 +1562,53 @@ func createAndCommit(t *testing.T, repo *git.Repository, path string, content []
 			t.Fatalf("os.MkdirAll failed: %v", err)
 		}
 	}
+	return commitChanges(t, repo, commitMsg, path)
+
+}
+
+// deleteAndCommit deletes a file in a repository and commits the change.
+func deleteAndCommit(t *testing.T, repo *git.Repository, path string, commitMsg string) *object.Commit {
+	t.Helper()
+	w, err := repo.Worktree()
+	if err != nil {
+		t.Fatalf("Worktree() failed: %v", err)
+	}
 
-	if _, err := w.Add(path); err != nil {
-		t.Fatalf("w.Add failed: %v", err)
+	fullPath := filepath.Join(w.Filesystem.Root(), path)
+	if err := os.Remove(fullPath); err != nil {
+		t.Fatalf("Remove() failed: %v", err)
+	}
+	return commitChanges(t, repo, commitMsg, path)
+}
+
+// renameAndCommit renames a file in a repository and commits the change.
+func renameAndCommit(t *testing.T, repo *git.Repository, fromPath, toPath string, commitMsg string) *object.Commit {
+	t.Helper()
+	w, err := repo.Worktree()
+	if err != nil {
+		t.Fatalf("Worktree() failed: %v", err)
+	}
+
+	fullFromPath := filepath.Join(w.Filesystem.Root(), fromPath)
+	fullToPath := filepath.Join(w.Filesystem.Root(), toPath)
+	if err := os.Rename(fullFromPath, fullToPath); err != nil {
+		t.Fatalf("Rename() failed: %v", err)
+	}
+	return commitChanges(t, repo, commitMsg, fromPath, toPath)
+}
+
+// commitChanges adds the specified paths to the repository and commits the change.
+func commitChanges(t *testing.T, repo *git.Repository, commitMsg string, paths ...string) *object.Commit {
+	t.Helper()
+	w, err := repo.Worktree()
+	if err != nil {
+		t.Fatalf("Worktree() failed: %v", err)
+	}
+
+	for _, path := range paths {
+		if _, err := w.Add(path); err != nil {
+			t.Fatalf("w.Add failed: %v", err)
+		}
 	}
 	hash, err := w.Commit(commitMsg, &git.CommitOptions{
 		Author: &object.Signature{Name: "Test", Email: "[email protected]"},
@@ -1589,6 +1643,14 @@ func setupRepoForChangedFilesTest(t *testing.T) (*LocalRepository, map[string]st
 	commit3 := createAndCommit(t, repo, "file2.txt", []byte("content3"), "commit 3")
 	commitHashes["commit 3"] = commit3.Hash.String()
 
+	// Commit 4 (delete file2.txt)
+	commit4 := deleteAndCommit(t, repo, "file2.txt", "commit 4")
+	commitHashes["commit 4"] = commit4.Hash.String()
+
+	// Commit 5 (rename file1.txt to file3.txt)
+	commit5 := renameAndCommit(t, repo, "file1.txt", "file3.txt", "commit 5")
+	commitHashes["commit 5"] = commit5.Hash.String()
+
 	return &LocalRepository{Dir: dir, repo: repo}, commitHashes
 }