interpret-trailers: add an option to unfold values

interpret-trailers: add an option to unfold values

The point of "--only-trailers" is to give a caller an output
that's easy for them to parse. Getting rid of the
non-trailer material helps, but we still may see more
complicated syntax like whitespace continuation. Let's add
an option to unfold any continuation, giving the output as a
single "key: value" line per trailer.

As a bonus, this could be used even without --only-trailers
to clean up unusual formatting in the incoming data.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>