Marking changes to be backported

[Glenn Morris]

Stefan Monnier wrote:

> I think it's too late to do that for Emacs-24.4.  But we should apply
> such a change to `emacs-24' after the 24.4 release, so that it will be
> included in the next release regardless if the next release is 25.1 or
> a 24.5 bugfix.

There needs to be an agreed convention to mark trunk changes that should
be backported to the release branch at some later date. "Remember to do
it" doesn't scale.

A note in the commit log would do, but it should have some kind of
standard format. But if we only realize after the commit was made,
some other method is needed. Either a file in admin, or perhaps a git
notes file.