notify: add option for deterministic message-id generation Copied from email by durin42: > Pierre-Yves asked offline why I asked a new option for this and why it > is not the default. Message-Id is supposed to be unique world-wide and > while it is desirable to have a deterministic mechanism for them for > creating follow-up emails, it needs organisational control for ensuring > the uniqueness. Differential Revision: https://phab.mercurial-scm.org/D6824