gpg-interface: provide access to the payload

In contrast to tag signatures, commit signatures are put into the
header, that is between the other header parts and commit messages.

Provide access to the commit content sans the signature, which is the
payload that is actually signed. Commit signature verification does the
parsing anyways, and callers may wish to act on or display the commit
object sans the signature.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.h

@@ -2,6 +2,7 @@
 #define GPG_INTERFACE_H
 
 struct signature_check {
+	char *payload;
 	char *gpg_output;
 	char *gpg_status;
 	char result; /* 0 (not checked),