Support serving tags via JSON format.

Tested manually locally. Feel free to suggest some testing, though I
didn't see a pre-existing RevisionServletTest in javatests to add to.

Issue: https://github.com/google/gitiles/issues/289
Bug: b/273569892
Change-Id: I90fb4b6e5eaa6b90d0e3267d491e99bdbe149004

java/com/google/gitiles/RevisionServlet.java

diff --git a/java/com/google/gitiles/RevisionServlet.java b/java/com/google/gitiles/RevisionServlet.java
index 3aeb352..37bb2b4 100644
--- a/java/com/google/gitiles/RevisionServlet.java
+++ b/java/com/google/gitiles/RevisionServlet.java
@@ -168,6 +168,9 @@
     try (RevWalk walk = new RevWalk(repo)) {
       DateFormatter df = new DateFormatter(getAccess(req), Format.DEFAULT);
       RevObject obj = walk.parseAny(view.getRevision().getId());
+      if (obj.getType() == OBJ_TAG) {
+        obj = ((RevTag) obj).getObject();
+      }
       switch (obj.getType()) {
         case OBJ_COMMIT:
           renderJson(