refactor(3p/nix): Apply clang-tidy's performance-* fixes r/789

This applies the performance fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

1 files changed, 1 insertions, 1 deletions

diff --git a/third_party/nix/src/nix-store/graphml.cc b/third_party/nix/src/nix-store/graphml.cc
index 6510b32881..a1a16cc8f2 100644
--- a/third_party/nix/src/nix-store/graphml.cc
+++ b/third_party/nix/src/nix-store/graphml.cc
@@ -37,7 +37,7 @@ static string makeNode(const ValidPathInfo& info) {
       (isDerivation(info.path) ? "derivation" : "output-path"));
 }
 
-void printGraphML(ref<Store> store, const PathSet& roots) {
+void printGraphML(const ref<Store>& store, const PathSet& roots) {
   PathSet workList(roots);
   PathSet doneSet;
   std::pair<PathSet::iterator, bool> ret;