Add "branch" information for builtins.fetchGit

This is the counterpart of nix-tools proposed change:
input-output-hk/nix-tools#99

Author: ismaelbouyaf

lib/stack-cache-generator.nix

@@ -58,6 +58,7 @@ let
         cp -r "${maybeCleanedSource}/." $TMP
         chmod -R +w $TMP
         substituteInPlace ${stackYaml} --replace "# nix-sha256:" "nix-sha256:"
+        substituteInPlace ${stackYaml} --replace "# nix-branch:" "nix-branch:"
         ${pkgs.lib.optionalString (fetchedResolver != null) ''
           substituteInPlace ${stackYaml} --replace "${resolver}" "${fetchedResolver}"
         ''}
@@ -90,9 +91,9 @@ concatMap (dep:
                   inherit (dep) url rev;
                   inherit sha256;
                 }
-                else builtins.fetchGit {
+                else builtins.fetchGit ({
                   inherit (dep) url rev;
-                };
+                } // pkgs.lib.optionalAttrs (dep ? branch && dep.branch != null) { ref = dep.branch; });
         in map (subdir: {
                 name = cabalName "${pkgsrc}/${subdir}";
                 inherit (dep) url rev;