[errorprone] Fix JavaUtilDate

Update to use PersonIdent#getWhenAsInstant to fix this error raised by
errorprone. Commit timestamps are stored with 1 second resolution.

See https://git-scm.com/docs/git-commit#Documentation/git-commit.txt-Gitinternalformat

Change-Id: Id97e1915fca64799d365184d0eec5afe98ba8637

java/com/google/gitiles/TimeCache.java

diff --git a/java/com/google/gitiles/TimeCache.java b/java/com/google/gitiles/TimeCache.java
index e1e55ce..4350d5c 100644
--- a/java/com/google/gitiles/TimeCache.java
+++ b/java/com/google/gitiles/TimeCache.java
@@ -65,7 +65,7 @@
               RevTag tag = (RevTag) o;
               PersonIdent ident = tag.getTaggerIdent();
               if (ident != null) {
-                return ident.getWhen().getTime() / 1000;
+                return ident.getWhenAsInstant().toEpochMilli() / 1000;
               }
               o = tag.getObject();
               walk.parseHeaders(o);

javatests/com/google/gitiles/TimeCacheTest.java

diff --git a/javatests/com/google/gitiles/TimeCacheTest.java b/javatests/com/google/gitiles/TimeCacheTest.java
index 8fe900a..2540c56 100644
--- a/javatests/com/google/gitiles/TimeCacheTest.java
+++ b/javatests/com/google/gitiles/TimeCacheTest.java
@@ -53,7 +53,7 @@
     repo = new TestRepository<>(new InMemoryRepository(new DfsRepositoryDescription("test")));
     walk = new RevWalk(repo.getRepository());
     cache = new TimeCache();
-    start = repo.getDate().getTime() / 1000;
+    start = repo.getInstant().toEpochMilli() / 1000;
   }
 
   private long getTime(ObjectId id) throws IOException {