Fix Eclipse warning: properly close DiffFormatter

Change-Id: Ic43f8498bff2e64ee0f56c2300eb3c562b8ed224

src/main/java/com/googlesource/gerrit/plugins/serverconfig/UnifiedDiffer.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/serverconfig/UnifiedDiffer.java b/src/main/java/com/googlesource/gerrit/plugins/serverconfig/UnifiedDiffer.java
index 96e363a..ecdad74 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/serverconfig/UnifiedDiffer.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/serverconfig/UnifiedDiffer.java
@@ -34,8 +34,9 @@
     EditList editList = algorithm.diff(RawTextComparator.DEFAULT, v0, v1);
 
     ByteArrayOutputStream os = new ByteArrayOutputStream();
-    DiffFormatter formatter = new DiffFormatter(os);
-    formatter.format(editList, v0, v1);
+    try (DiffFormatter formatter = new DiffFormatter(os)) {
+      formatter.format(editList, v0, v1);
+    }
 
     return os.toString(CHARSET_NAME);
   }