lib/commit: Check that dirent is a directory before cleaning

lib/commit: Check that dirent is a directory before cleaning

I've only noticed this by inspection. But I think it's possible for
`cleanup_txn_dir` to get called with the `staging-...-lock` file since
it matches the prefix.

Make the checking here stronger by verifying that it's a directory. If
it's not a directory (lockfile), then follow the default pruning expiry
logic so that we still cleanup stray lockfiles eventually.