Include Co-authored-by entries in AUTHORS.

Co-authored-by lines are the way we've decided to indicated shared
authorship of a patch, so content from them should be included in
the generated AUTHORS file.

Fixes bug 1158319.

Change-Id: I9dacf78c01f3ad74e696f16a7aa39edb98e8d185
Merge "Include Co-authored-by entries in AUTHORS."