simplemerge: leverage pycompat function to convert byte string to set

simplemerge: fix function name that tests if ctx is not null revision

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