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>

Documentation/config.txt

@@ -1832,6 +1832,11 @@ gpg.format::
 	Specifies which key format to use when signing with `--gpg-sign`.
 	Default is "openpgp", that is also the only supported value.
 
+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`.
+
 gui.commitMsgWidth::
 	Defines how wide the commit message window is in the
 	linkgit:git-gui[1]. "75" is the default.

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) {