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

In theory it's easy for any reader who wants to parse
trailers to do so. But there are a lot of subtle corner
cases around what counts as a trailer, when the trailer
block begins and ends, etc. Since interpret-trailers already
has our parsing logic, let's let callers ask it to just
output the trailers.

They still have to parse the "key: value" lines, but at
least they can ignore all of the other corner cases.

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

trailer.h

@@ -25,6 +25,7 @@ struct trailer_info {
 struct process_trailer_options {
 	int in_place;
 	int trim_empty;
+	int only_trailers;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}