interpret-trailers: add an option to show only existing trailers

interpret-trailers: add an option to show only existing trailers

It can be useful to invoke interpret-trailers for the
primary purpose of parsing existing trailers. But in that
case, we don't want to apply existing ifMissing or ifExists
rules from the config. Let's add a special mode where we
avoid applying those rules. Coupled with --only-trailers,
this gives us a reasonable parsing tool.

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