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