pretty: allow showing specific trailers

Adds a new "key=X" option to "%(trailers)" which will cause it to only
print trailer lines which match any of the specified keys.

Signed-off-by: Anders Waldenborg <anders@0x63.nu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trailer.h

@@ -72,6 +72,8 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int (*filter)(const struct strbuf *, void *);
+	void *filter_data;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}