feat(services/sync-gcsr): Add git synchronisation helper

Adds a tiny program that keeps a checkout of a remote git repository
in sync with a folder on the local filesystem.

This is going to be used to mirror the GCSR repository onto a local
disk for cgit serving.

services/sync-gcsr/default.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+pkgs.buildGo.program {
+  name = "sync-gcsr";
+  srcs = [ ./main.go ];
+
+  deps = with pkgs.third_party; map (p: p.gopkg) [
+    gopkgs."gopkg.in".src-d.go-git
+  ];
+}