search --output=files: Output all filenames for each matching message

Messages in the database can have multiple files associated with a
single message-id, but until now only one filename for each message
has been reported by "notmuch search --output=files"

Signed-off-by: Mark Anderson <ma.skies@gmail.com>
(cherry picked from commit d752509abfd7697d2f8a1c5be6f96f0df4f0299d)