Optimze backtracer operator=

include/spdlog/details/backtracer-inl.h

@@ -26,7 +26,7 @@ SPDLOG_INLINE backtracer &backtracer::operator=(backtracer other)
 {
     std::lock_guard<std::mutex> lock(mutex_);
     enabled_ = other.enabled();
-    messages_ = other.messages_;
+    messages_ = std::move(other.messages_);
     return *this;
 }