git: add config option for gpg.format

git provides three options for signing key format, openpgp(default),
ssh, and x509. This commit provides the option to specify one of the
other two available formats.

modules/programs/git.nix

@@ -38,6 +38,17 @@ let
         defaultText = "\${pkgs.gnupg}/bin/gpg2";
         description = "Path to GnuPG binary to use.";
       };
+
+      gpgFormat = mkOption {
+        type = types.str;
+        default = "openpgp";
+        defaultText = "openpgp";
+        description = ''
+          Specifies which key format to use when signing with
+          --gpg-sign. Default is "openpgp". Other possible values
+          are "x509", "ssh".
+        '';
+      };
     };
   };
 
@@ -421,6 +432,7 @@ in {
         commit.gpgSign = mkDefault cfg.signing.signByDefault;
         tag.gpgSign = mkDefault cfg.signing.signByDefault;
         gpg.program = cfg.signing.gpgPath;
+        gpg.format = cfg.signing.gpgFormat;
       };
     })
 

tests/modules/programs/git/git-expected.conf

@@ -38,6 +38,7 @@
 	smudge = "git-lfs smudge -- %f"
 
 [gpg]
+	format = "ssh"
 	program = "path-to-gpg"
 
 [interactive]

tests/modules/programs/git/git.nix

@@ -55,6 +55,7 @@ in {
           }
         ];
         signing = {
+          gpgFormat = "ssh";
           gpgPath = "path-to-gpg";
           key = "00112233445566778899AABBCCDDEEFF";
           signByDefault = true;