git: decode node IDs back into Python strings (issue6349)
db.text_factory = bytes, so the database contains only strings. The object IDs
we get from pygit2 are Python strings. b'foo' != 'foo'
This change allows the "don't reindex" optimization to work by allowing the
"cur_cache_heads == cache_heads" comparison a few lines down to succeed.
Differential Revision: https://phab.mercurial-scm.org/D8622