Add path flakeref variant

Unlike file://<path>, this allows the path to be a dirty Git tree, so

  nix build /path/to/flake:attr

is a convenient way to test building a local flake.

src/libexpr/primops/fetchGit.hh

@@ -11,7 +11,7 @@ struct GitInfo
     Path storePath;
     std::string rev;
     std::string shortRev;
-    uint64_t revCount = 0;
+    std::optional<uint64_t> revCount;
 };
 
 GitInfo exportGit(ref<Store> store, const std::string & uri,