Convert to auto-closeable Repository and RevWalk

Change-Id: Ia2964f4e7fbf76948059b37ab47c07baeb6db78f

src/main/java/com/googlesource/gerrit/plugins/hooks/util/CommitMessageFetcher.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/util/CommitMessageFetcher.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/util/CommitMessageFetcher.java
index a04b175..02ea502 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/util/CommitMessageFetcher.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/util/CommitMessageFetcher.java
@@ -25,15 +25,11 @@
   }
 
   public String fetch(String projectName, String commitId) throws IOException {
-    final Repository repo =
-        repoManager.openRepository(new NameKey(projectName));
-    try {
-      RevWalk revWalk = new RevWalk(repo);
-      RevCommit commit = revWalk.parseCommit(ObjectId.fromString(commitId));
-
-      return commit.getFullMessage();
-    } finally {
-      repo.close();
+    try (Repository repo = repoManager.openRepository(new NameKey(projectName))) {
+      try (RevWalk revWalk = new RevWalk(repo)) {
+        RevCommit commit = revWalk.parseCommit(ObjectId.fromString(commitId));
+        return commit.getFullMessage();
+      }
     }
   }