Add toString() for PrepareDepsOfTargetsUnderDirectoryValue key.

--
MOS_MIGRATED_REVID=101510850
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/PrepareDepsOfTargetsUnderDirectoryValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/PrepareDepsOfTargetsUnderDirectoryValue.java
index 353bd5e..61096fb 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/PrepareDepsOfTargetsUnderDirectoryValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/PrepareDepsOfTargetsUnderDirectoryValue.java
@@ -13,6 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.skyframe;
 
+import com.google.common.base.MoreObjects;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
@@ -150,5 +151,13 @@
     public int hashCode() {
       return Objects.hash(recursivePkgKey, filteringPolicy);
     }
+
+    @Override
+    public String toString() {
+      return MoreObjects.toStringHelper(PrepareDepsOfTargetsUnderDirectoryKey.class)
+              .add("pkg-key", recursivePkgKey)
+              .add("filtering policy", filteringPolicy)
+              .toString();
+    }
   }
 }