bug#20400: wrong author line in generated ChangeLog

[Glenn Morris]

Package: emacs
Version: 25.0.50

`make ChangeLog' gives me a file containing this:

2015-04-19  Steve Purcell  <steve@sanityinc.com>
            Steve Purcell  <steve@sanityinc.com>

Presumably this is due to a mistake in
http://lists.gnu.org/archive/html/emacs-diffs/2015-04/msg00308.html

Three things to fix IMO:

1) That specific ChangeLog entry

2) The description of Co-authored-by in CONTRIBUTE seems misleading. I
guess it should say something along the lines of "If the commit has
authors other than the one listed in the Author: header (ie, the one
passed to git commit --author")..." rather than "other than yourself".

3) The ChangeLog generation script should avoid listing literally
identical authors twice.