Notedb: Move individual note parsing into a new class

We would like to store additional information besides inline comments
in notes. As a first step towards doing so, encapsulate parsing state
into a new class, RevisionNote. This class reads the note data once
and can parse it into multiple fields (but for now still one).

When updating the note, we copy the immutable RevisionNote into a
mutable RevisionNoteBuilder, which knows how to write itself out to a
note. This shifts a little bit of work from CommentsInNotesUtil to
ChangeUpdate.

Change-Id: Ia55888b043a5624b81a3fdde733363cd1f1a5aee

gerrit-server/src/main/java/com/google/gerrit/server/notedb/RevisionNote.java

@@ -0,0 +1,40 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
+
+import com.google.common.collect.ImmutableList;
+import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.PatchLineComment;
+
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
+
+import java.io.IOException;
+
+class RevisionNote {
+  static final int MAX_NOTE_SZ = 25 << 20;
+
+  final ImmutableList<PatchLineComment> comments;
+
+  RevisionNote(Change.Id changeId, ObjectReader reader, ObjectId noteId)
+      throws ConfigInvalidException, IOException {
+    byte[] bytes = reader.open(noteId, OBJ_BLOB).getCachedBytes(MAX_NOTE_SZ);
+    comments = ImmutableList.copyOf(CommentsInNotesUtil.parseNote(
+        bytes, changeId, PatchLineComment.Status.PUBLISHED));
+  }
+}