Detect notes containing committer info that doesn't match commit
Use committer information from mercurial extra data

As a side effect, this fixes the "corruptions" in the canonical Mercurial
repository on the three changesets with a whitespace after the timezone.
Dangling metadata and notes are not corruptions
Only emit "done" fast-import message when closing the store
Don't pretend remote-helper options are supported
Only request remote heads once