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

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>