Create a struct that holds the format details for the supported formats.
At the moment that is still just "openpgp". This commit prepares for the
introduction of more formats, that might use other programs and match
other signatures.
Signed-off-by: Henning Schild <henning.schild@siemens.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>