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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors