DateFormatter: Reduce visibility of constructor used for tests

DateFormatter(Optional<TimeZone> fixedTz, Format format) is only used in
tests. Reduce its visibility and annotate as @VisibleForTesting.

Change-Id: I2b62d91669a86f2510e89869c377413b3a7bd197

java/com/google/gitiles/DateFormatter.java

diff --git a/java/com/google/gitiles/DateFormatter.java b/java/com/google/gitiles/DateFormatter.java
index b14483f..69e8a86 100644
--- a/java/com/google/gitiles/DateFormatter.java
+++ b/java/com/google/gitiles/DateFormatter.java
@@ -14,6 +14,7 @@
 
 package com.google.gitiles;
 
+import com.google.common.annotations.VisibleForTesting;
 import java.io.IOException;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
@@ -63,7 +64,8 @@
   private final Optional<TimeZone> fixedTz;
   private final Format format;
 
-  public DateFormatter(Optional<TimeZone> fixedTz, Format format) {
+  @VisibleForTesting
+  protected DateFormatter(Optional<TimeZone> fixedTz, Format format) {
     this.fixedTz = fixedTz;
     this.format = format;
   }