Allow configuring git

lib/modules/git.nix

@@ -0,0 +1,41 @@
+{ lib, ... }:
+  let
+    inherit (lib)
+      mkOption
+      types
+    ;
+
+    gitOptions = {
+      defaultBranch = mkOption {
+        type = types.nullOr types.str;
+        description = "The name of the default branch in git.";
+        default = null;
+      };
+
+      flow = {
+        mainBranch = mkOption {
+          type = types.nullOr types.str;
+          description = "The name of the stable branch in git flow.";
+          default = null;
+        };
+
+        devBranch = mkOption {
+          type = types.nullOr types.str;
+          description = "The name of the development branch in git flow.";
+          default = null;
+        };
+      };
+
+      aliases = mkOption {
+        type = types.attrsOf types.str;
+        description = "Git command aliases to install.";
+        default = {};
+      };
+    };
+  in {
+    options = {
+      valhalla = {
+        git = gitOptions;
+      };
+    };
+  }

lib/modules/valhalla.nix

@@ -8,6 +8,7 @@
     cfg = config.valhalla;
   in {
     imports = [
+      ./git.nix
       ./i18n.nix
       ./partition.nix
     ];