Make single_node_merge more clear

When reading logs from multiple nodes trying to figure out what
happened, it can be rather unclear.  Update the prints to help.

Change-Id: I285da46f0fa240a687cfaae1af01fa882641f3ca
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/events/logging/single_node_merge.cc b/aos/events/logging/single_node_merge.cc
index 0fa3567..a34e4c2 100644
--- a/aos/events/logging/single_node_merge.cc
+++ b/aos/events/logging/single_node_merge.cc
@@ -74,7 +74,8 @@
     if (m == nullptr) {
       break;
     }
-    std::cout << config->nodes()
+    std::cout << "on " << FLAGS_node << " from "
+              << config->nodes()
                      ->Get(configuration::GetNodeIndex(
                          config, config->channels()
                                      ->Get(m->channel_index)