Fix data race on prefetcher.

Individual file prefetches within a single prefetchFiles() can race against
each other, so they must synchronize when writing to the DirectoryContext.

src/main/java/com/google/devtools/build/lib/remote/AbstractActionInputPrefetcher.java

@@ -60,7 +60,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Deque;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -95,7 +94,7 @@ public abstract class AbstractActionInputPrefetcher implements ActionInputPrefet
 
   /** Keeps track of output directories written to by a single prefetcher call. */
   private final class DirectoryContext {
-    private final HashSet<Path> dirs = new HashSet<>();
+    private final Set<Path> dirs = Sets.newConcurrentHashSet();
 
     /**
      * Adds to the set of directories written to by the prefetcher call associated with this