gpg-interface: introduce new signature format "x509" using gpgsm

This commit allows git to create and check x509 type signatures using
gpgsm.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1830,12 +1830,13 @@ gpg.program::
 
 gpg.format::
 	Specifies which key format to use when signing with `--gpg-sign`.
-	Default is "openpgp", that is also the only supported value.
+	Default is "openpgp" and another possible value is "x509".
 
 gpg.<format>.program::
 	Use this to customize the program used for the signing format you
 	chose. (see `gpg.program` and `gpg.format`) `gpg.program` can still
-	be used as a legacy synonym for	`gpg.openpgp.program`.
+	be used as a legacy synonym for	`gpg.openpgp.program`. The default
+	value for `gpg.x509.program` is "gpgsm".
 
 gui.commitMsgWidth::
 	Defines how wide the commit message window is in the

gpg-interface.c

@@ -24,11 +24,23 @@ static const char *openpgp_sigs[] = {
 	NULL
 };
 
+static const char *x509_verify_args[] = {
+	NULL
+};
+static const char *x509_sigs[] = {
+	"-----BEGIN SIGNED MESSAGE-----",
+	NULL
+};
+
 static struct gpg_format gpg_format[] = {
 	{ .name = "openpgp", .program = "gpg",
 	  .verify_args = openpgp_verify_args,
 	  .sigs = openpgp_sigs
 	},
+	{ .name = "x509", .program = "gpgsm",
+	  .verify_args = x509_verify_args,
+	  .sigs = x509_sigs
+	},
 };
 
 static struct gpg_format *use_format = &gpg_format[0];
@@ -192,6 +204,9 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
 		fmtname = "openpgp";
 
+	if (!strcmp(var, "gpg.x509.program"))
+		fmtname = "x509";
+
 	if (fmtname) {
 		fmt = get_format_by_name(fmtname);
 		return git_config_string(&fmt->program, var, value);