Log SkyKeys detected changed across builds

Similar to previous approach, except diff-awareness agnostic. Log
SkyKeys as is for simplicity- this is all for human consumption anyway.

--
MOS_MIGRATED_REVID=96428248
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/FilesystemValueChecker.java b/src/main/java/com/google/devtools/build/lib/skyframe/FilesystemValueChecker.java
index 923cc30..d2f4a06 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/FilesystemValueChecker.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/FilesystemValueChecker.java
@@ -371,7 +371,7 @@
     }
 
     @Override
-    public Iterable<SkyKey> changedKeysWithoutNewValues() {
+    public Collection<SkyKey> changedKeysWithoutNewValues() {
       return concurrentDirtyKeysWithoutNewValues;
     }