git: consistently use str for parents when rebuilding the index database The tests show no changes, but when these values are overwritten shortly after when a git commit object is available, that uses str. It seems better to use that for consistency. It does materially affect the database though, because the old value stored was `X'3030..3030'` and is now '00..00' when the changelog table is dumped in sqlite3. There is one query that specifies the parents, but it passes the non null hashes as str, so it worked as expected. That likely explains the lack of test changes. Differential Revision: https://phab.mercurial-scm.org/D10527