Fix resource leak flagged by infer static analyzer

Change-Id: Ia14f5ba8f245b93e4b0febae53c7d6888faf3b05

gitiles-servlet/src/main/java/com/google/gitiles/ReadmeHelper.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/ReadmeHelper.java b/gitiles-servlet/src/main/java/com/google/gitiles/ReadmeHelper.java
index e6c5736..468b26e 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/ReadmeHelper.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/ReadmeHelper.java
@@ -60,11 +60,12 @@
       throws MissingObjectException, IncorrectObjectTypeException, CorruptObjectException,
           IOException {
     if (config.render) {
-      TreeWalk tw = new TreeWalk(reader);
-      tw.setRecursive(false);
-      tw.addTree(tree);
-      while (tw.next() && !isPresent()) {
-        considerEntry(tw);
+      try (TreeWalk tw = new TreeWalk(reader)) {
+        tw.setRecursive(false);
+        tw.addTree(tree);
+        while (tw.next() && !isPresent()) {
+          considerEntry(tw);
+        }
       }
     }
   }