mergiraf: init module (#6633)

This adds git integration for mergiraf. It sets up git to use mergiraf
to automatically resolve merge conflicts.

modules/programs/mergiraf.nix

@@ -0,0 +1,31 @@
+{ pkgs, config, lib, ... }:
+let
+  inherit (lib)
+    mkEnableOption mkPackageOption types literalExpression mkIf maintainers;
+  cfg = config.programs.mergiraf;
+  mergiraf = "${cfg.package}/bin/mergiraf";
+in {
+  meta.maintainers = [ maintainers.bobvanderlinden ];
+
+  options = {
+    programs.mergiraf = {
+      enable = mkEnableOption "mergiraf";
+      package = mkPackageOption pkgs "mergiraf" { };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = [ cfg.package ];
+
+    programs.git = {
+      attributes = [ "* merge=mergiraf" ];
+      extraConfig = {
+        merge.mergiraf = {
+          name = "mergiraf";
+          driver =
+            "${mergiraf} merge --git %O %A %B -s %S -x %X -y %Y -p %P -l %L";
+        };
+      };
+    };
+  };
+}