notify: optional mail threading based on obsmarker
When notify.reply is set and a changeset has a predecessor in the
repository, include In-Reply-To pointing to the message-id that would
have been generated for the oldest predecessor. This allows mail
threading like Phabricator for common cases like rebasing changes, but
will be optimal for cases like folding.
Differential Revision: https://phab.mercurial-scm.org/D8172