[MERGE] Support multiple authors for a revision

[James Westby]

Hi,

Here's the changes suggested by John to allow this.

>From the commit message:

> Allow specifying multiple authors for a revision.
>   
>   The revision property "author" is now a "\n"-separated list of authors.
>   
>   --author can be specified multiple times to populate that list at commit
>   time.
>   
>   get_apparent_author() is deprecated in favour of get_apparent_authors(),
>   which will return a list.
>   
>   Some things (e.g. annotate) still just use the first item from that list,
>   but there is now support for other code to use all of the authors.


Thanks,

James