RepoUpdater: Make field final, remove #toString()

Minor style changes suggested by internal tools.

Change-Id: I0c9022da4f16e57c50d4b63fb95e8d2acecdfeaf
diff --git a/java/com/googlesource/gerrit/plugins/supermanifest/RepoUpdater.java b/java/com/googlesource/gerrit/plugins/supermanifest/RepoUpdater.java
index a0fb237..8f899bd 100644
--- a/java/com/googlesource/gerrit/plugins/supermanifest/RepoUpdater.java
+++ b/java/com/googlesource/gerrit/plugins/supermanifest/RepoUpdater.java
@@ -28,7 +28,7 @@
 
 class RepoUpdater implements SubModuleUpdater {
 
-  static String SUPERMANIFEST_STAMP = ".supermanifest";
+  static final String SUPERMANIFEST_STAMP = ".supermanifest";
 
   PersonIdent serverIdent;
 
@@ -61,9 +61,7 @@
         .setInputStream(manifestStream)
         .addToDestination(
             SUPERMANIFEST_STAMP,
-            String.format(
-                "%s %s %s",
-                c.getSrcRepoKey().toString(), srcRef, srcRepo.resolve(srcRef).getName()))
+            String.format("%s %s %s", c.getSrcRepoKey(), srcRef, srcRepo.resolve(srcRef).getName()))
         .setRecommendShallow(true)
         .setRemoteReader(reader)
         .setTargetURI(c.getDestRepoKey().toString())