trailer: add interface for iterating over commit trailers

trailer: add interface for iterating over commit trailers

The trailer code knows how to parse out the trailers and re-format them,
but there's no easy way to iterate over the trailers (you can use
trailer_info, but you have to then do a bunch of extra parsing).

Let's add an iteration interface that makes this easy to do.

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