fix bug: fetchPackageRepo (#2327)

Author: kgtkr

lib/call-cabal-project-to-nix.nix

@@ -193,10 +193,11 @@ let
             then fetchgit { inherit (repoData) url sha256; rev = repoData.rev or repoData.ref; }
             else
               let drv = builtins.fetchGit
-                { inherit (repoData) url ; ref = repoData.ref or null; }
-                # fetchGit does not accept "null" as rev, so when it's null
-                # we have to omit the argument completely.
-                // pkgs.lib.optionalAttrs (repoData ? rev) { inherit (repoData) rev; };
+                ({ inherit (repoData) url ; }
+                  # fetchGit does not accept "null" as rev and ref, so when it's null
+                  # we have to omit the argument completely.
+                  // pkgs.lib.optionalAttrs (repoData ? ref) { inherit (repoData) ref; }
+                  // pkgs.lib.optionalAttrs (repoData ? rev) { inherit (repoData) rev; });
               in __trace "WARNING: No sha256 found for source-repository-package ${repoData.url} ref=${repoData.ref or "(unspecified)"} rev=${repoData.rev or "(unspecified)"} download may fail in restricted mode (hydra)"
                 (__trace "Consider adding `--sha256: ${hashPath drv}` to the ${cabalProjectFileName} file or passing in a sha256map argument"
                  drv);