pretty format %(trailers): add a "key_value_separator"

Add a "key_value_separator" option to the "%(trailers)" pretty format,
to go along with the existing "separator" argument. In combination
these two options make it trivial to produce machine-readable (e.g. \0
and \0\0-delimited) format output.

As elaborated on in a previous commit which added "keyonly" it was
needlessly tedious to extract structured data from "%(trailers)"
before the addition of this "key_value_separator" option. As seen by
the test being added here extracting this data now becomes trivial.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trailer.h

@@ -74,6 +74,7 @@ struct process_trailer_options {
 	int key_only;
 	int value_only;
 	const struct strbuf *separator;
+	const struct strbuf *key_value_separator;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };