gpg-interface: introduce new config to select per gpg format program

Supporting multiple signing formats we will have the need to configure a
custom program each. Add a new config value to cater for that.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/gpg-interface.c b/gpg-interface.c
index 51cad90..a158f08 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -189,7 +189,7 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
-	if (!strcmp(var, "gpg.program"))
+	if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
 		fmtname = "openpgp";
 
 	if (fmtname) {