Add 'Parent $x' options to diff for merge commits

Add 'Parent $x' options to the diff base drop down for merge commits.
This way users are able to see changes brought by merge commit in
addition to the merge conflicts resolution.

Two main challenges of this change are:
* server: enable commenting on a Parent-N of a merge commit
* client: pass around the diff-base info which for merge commits can be:
  Auto-Merge, Parent-1, Parent-2, ... , Parent-N

Currently the patch_comments.side field uses two values:
1 = REVISION
0 = PARENT

For non-merge commits, side == 0 means comment of the Base (the parent)
of this patch-set.  For a merge commit, side == 0 means comment on the
Auto-Merge of this (merge) patch-set.

This change uses side == -N to store comment on the Parent-N of this (merge)
patch-set. For Parent-1 the side is -1, for Parent-2 the side is -2, etc..
This avoids need for a schema migration.

In NoteDb the parent number is stored in a new field: "Parent-number: 1".

CommentInfo was extended with the new "parent" field which is 1-based
parent number.  Some REST API endpoints expose a new --parent option to
enable referencing a specific parent of a merge commit.

On the client side we typically pass around two PatchSet.Id's: base and
revision to represent the state of the UI. The base had two meanings:
* when it was non-null it was representing another patch-set
* when it was null it represented the parent for a non-merge commit and
  the auto-merge for a merge commit.

For a merge commit we need to also pass around Parent-N as (diff) base for a
merge commit. To keep the number of changes minimal this change proposes
(re)using the base patch-set for that where its patch-set number is negative.
Therefore, when base is not null but its patch-set number is negative (-N) it
represents Parent-N of the patch-set represented by the revision patch-set.
This is also expressed in the client URL token which uses negative numbers
for parent(s) of a merge commit. For example: -1..2 represents comparison
of the second patch-set against its first parent. -2..2 would represent
comparison of the patch-set 2 against its second parent.

I experimented with introducing a DiffBase class which would be a combination
of a base patch-set plus a parent number:

  class DiffBase {
    PatchSet.Id base;
    int parent;
  }

which would avoid the ugliness of using a base with negative patch-set number.
However, this produced neither a smaller nor a more elegant change. The number
of changed files actually increased and, still, all places where base is used
had to handle the case where base is null and parent is greater than zero.

Bug: Issue 106
Change-Id: If0d7b13fad9051ec2943f6d51c99e84f7d2af708
Also-by: Dariusz Luksza <dariusz@luksza.org>
Also-by: Saša Živkov <zivkov@gmail.com>

gerrit-server/src/main/java/com/google/gerrit/server/change/FileInfoJson.java

@@ -14,6 +14,8 @@
 
 package com.google.gerrit.server.change;
 
+import static com.google.gerrit.server.util.GitUtil.getParent;
+
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.client.DiffPreferencesInfo.Whitespace;
 import com.google.gerrit.extensions.common.FileInfo;
@@ -21,6 +23,7 @@ import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.Patch;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.reviewdb.client.RevId;
+import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.patch.PatchList;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchListEntry;
@@ -29,17 +32,24 @@ import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 
+import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Repository;
 
+import java.io.IOException;
 import java.util.Map;
 import java.util.TreeMap;
 
 @Singleton
 public class FileInfoJson {
   private final PatchListCache patchListCache;
+  private final GitRepositoryManager repoManager;
 
   @Inject
-  FileInfoJson(PatchListCache patchListCache) {
+  FileInfoJson(
+      PatchListCache patchListCache,
+      GitRepositoryManager repoManager) {
+    this.repoManager = repoManager;
     this.patchListCache = patchListCache;
   }
 
@@ -54,6 +64,22 @@ public class FileInfoJson {
         ? null
         : ObjectId.fromString(base.getRevision().get());
     ObjectId b = ObjectId.fromString(revision.get());
+    return toFileInfoMap(change, a, b);
+  }
+
+  Map<String, FileInfo> toFileInfoMap(Change change, RevId revision, int parent)
+      throws RepositoryNotFoundException, IOException,
+          PatchListNotAvailableException {
+    ObjectId b = ObjectId.fromString(revision.get());
+    ObjectId a;
+    try (Repository git = repoManager.openRepository(change.getProject())) {
+      a = getParent(git, b, parent);
+    }
+    return toFileInfoMap(change, a, b);
+  }
+
+  private Map<String, FileInfo> toFileInfoMap(Change change,
+      ObjectId a, ObjectId b) throws PatchListNotAvailableException {
     PatchList list = patchListCache.get(
         new PatchListKey(a, b, Whitespace.IGNORE_NONE), change.getProject());