Add safe immutable directory promotion

internal/fileops/rename_noreplace_linux.go

@@ -0,0 +1,9 @@
+//go:build linux
+
+package fileops
+
+import "golang.org/x/sys/unix"
+
+func renameDirectoryNoReplace(src, dst string) error {
+	return unix.Renameat2(unix.AT_FDCWD, src, unix.AT_FDCWD, dst, unix.RENAME_NOREPLACE)
+}