LocalLargeFileRepository#getContentPath: Avoid String concatentation

The CONTENT_PATH constant is not used ouside the class since
change I4339d714f. Make it private.

Also rename it to CONSTANT_PATH_TEMPLATE and use String.format to
generate the content path from the backend name.

Change-Id: If12d8f3c6350155d867f1b52ff040b2f00e89119
diff --git a/src/main/java/com/googlesource/gerrit/plugins/lfs/fs/LocalLargeFileRepository.java b/src/main/java/com/googlesource/gerrit/plugins/lfs/fs/LocalLargeFileRepository.java
index a48d4bc..812af2f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/lfs/fs/LocalLargeFileRepository.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/lfs/fs/LocalLargeFileRepository.java
@@ -41,7 +41,7 @@
     LocalLargeFileRepository create(LfsBackend backendConfig);
   }
 
-  public static final String CONTENT_PATH = "content";
+  private static final String CONTENT_PATH_TEMPLATE = "content/%s/";
   private static final int DEFAULT_TIMEOUT = 10; // in seconds
 
   private final String servletUrlPattern;
@@ -93,7 +93,7 @@
   }
 
   private static String getContentPath(LfsBackend backend) {
-    return CONTENT_PATH + "/" + backend.name() + "/";
+    return String.format(CONTENT_PATH_TEMPLATE, backend.name());
   }
 
   private static Path getOrCreateDataDir(