Support JSON view of commits in RevisionServlet

Example output:

$ curl 'http://localhost:8080/gitiles/+/1040ab8d5861e2804f725a936ca9b6f3be6f6239?format=JSON'
)]}'
{
  "commit": "1040ab8d5861e2804f725a936ca9b6f3be6f6239",
  "parents": [
    "0ad024fb5aa3e4994ede801428b4e30ba06a837d"
  ],
  "author": {
    "name": "Dave Borowitz",
    "email": "dborowitz@google.com",
    "time": "Sun Mar 16 13:49:37 2014 -0700"
  },
  "committer": {
    "name": "Dave Borowitz",
    "email": "dborowitz@google.com",
    "time": "Sun Mar 16 13:49:37 2014 -0700"
  },
  "message": "Use CommitData to produce CommitJsonData\n\nChange-Id: I7261b0c40e97119c32627c4ea89038bbe174925f\n",
  "tree_diff": [
    {
      "type": "modify",
      "old_id": "87f3456c40f24662d28411fda9b2cf05b82bf007",
      "old_mode": 33188,
      "old_path": "gitiles-servlet/src/main/java/com/google/gitiles/CommitJsonData.java",
      "new_id": "07fbd1e0eb753aa41dee53c758ced595f18f67bf",
      "new_mode": 33188,
      "new_path": "gitiles-servlet/src/main/java/com/google/gitiles/CommitJsonData.java"
    },
    {
      "type": "modify",
      "old_id": "aa6bd34861cf2f49abd357edff0b2f3ff354aa91",
      "old_mode": 33188,
      "old_path": "gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java",
      "new_id": "e6ae4c2062bfe3dfb6f059f282e90af64bbd10d3",
      "new_mode": 33188,
      "new_path": "gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java"
    }
  ]
}

Change-Id: I29b76ce331bd6876476e3ddd56f2f484450b7e87

gitiles-servlet/src/main/java/com/google/gitiles/CommitData.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/CommitData.java b/gitiles-servlet/src/main/java/com/google/gitiles/CommitData.java
index 1c118dd..0f614e6 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/CommitData.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/CommitData.java
@@ -22,7 +22,9 @@
 import com.google.common.base.Predicate;
 import com.google.common.collect.FluentIterable;
 import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Iterables;
 import com.google.common.collect.Ordering;
+import com.google.common.collect.Sets;
 
 import org.eclipse.jgit.diff.DiffEntry;
 import org.eclipse.jgit.diff.DiffFormatter;
@@ -70,6 +72,10 @@
     TREE,
     TREE_URL,
     URL;
+
+    static ImmutableSet<Field> setOf(Iterable<Field> base, Field... fields) {
+      return Sets.immutableEnumSet(Iterables.concat(base, Arrays.asList(fields)));
+    }
   }
 
   static class DiffList {