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
Remove unused GitHgStore._manifests_by_changeset
Use rev-list --stdin in findcommon()
Refresh refs/remote-hg/tip if it was changed
Initial support for push, purposefully limited to local repositories