[MERGE] Support multiple authors for a revision (#185772)

[Ben Finney]

Ian Clatworthy <ian.clatworthy at internode.on.net> writes:

> James Westby wrote:
> > Newlines weren't excluded before though, so it's a theoretical
> > problem already, it would be perfectly possible to cause the
> > problems already. This change just makes it much more likely.
> > 
> > I'm not sure if this is something that we should worry about?
> 
> I expect this will break things so we ought to consider it
> carefully. For example, fast-export/fast-import will need tweaks to
> handle the newline (otherwise it will spit out an illegal data
> stream).