mailmap: Canonicalize Rein Zustand like in zulip/zulip.

Author: timabbott

.mailmap

@@ -3,6 +3,7 @@ Anders Kaseorg <anders@zulip.com> <anders@zulipchat.com>
 Anders Kaseorg <anders@zulip.com> <andersk@mit.edu>
 Jessica McKellar <jesstess@zulip.com> <jesstess@humbughq.com>
 Luke Faraone <lfaraone@zulip.com> <lfaraone@humbughq.com>
+Rein Zustand (rht) <rhtbot@protonmail.com>
 Rishi Gupta <rishig@zulip.com> <rishig@zulipchat.com>
 Steve Howell <showell@zulip.com> <showell30@yahoo.com>
 Steve Howell <showell@zulip.com> <showell@zulipchat.com>