gitlog-to-changelog: support multi-author commits.

gitlog-to-changelog: support multi-author commits.

The FSF cares about keeping track of all authors of patches to its
projects, but Git doesn't provide obvious support for multi-author
changesets. Consensus seems to be forming around the use of extra
Signed-off-by inspired lines in the log message formatted as
`Co-authored-by: A U Thor <email@example.com>' for round-tripping
multi-author commits between version control systems.
* gitlog-to-changelog: Extract `Co-authored-by:' lines from the git
log message and output in standard ChangeLog multi-author format.
Reported by Peter Rosin <peda@lysator.liu.se>