Resolve source operations from the Git checkout root

internal/git/source.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -30,6 +31,24 @@ type TaggedSource struct {
 	TagObjectType string
 }
 
+// RepositoryRoot resolves the containing checkout, including independent
+// worktrees and directories reached through filesystem aliases.
+func RepositoryRoot(ctx context.Context, directory string) (string, error) {
+	if directory == "" {
+		directory = "."
+	}
+	root, err := run(ctx, directory, nil, "rev-parse", "--show-toplevel")
+	if err != nil {
+		return "", fmt.Errorf("resolve Git repository root: %w", err)
+	}
+	root = strings.TrimSuffix(root, "\n")
+	resolved, err := filepath.EvalSymlinks(root)
+	if err != nil {
+		return "", fmt.Errorf("resolve Git repository directory: %w", err)
+	}
+	return resolved, nil
+}
+
 // CaptureHead records the checkout commit before identity derived from files
 // in that commit is inspected.
 func CaptureHead(ctx context.Context, repositoryDir string) (string, error) {