MergeSuperSet: Handle changes that exist in index but not in NoteDb gracefully

If a system has multiple primary nodes it can happen that the index
returns changes for a topic that don't exist in NoteDb yet which then
causes the SubmittedTogether REST endpoint to fail on permission checks:

1. Change 1 with topic foo is created and replicated to all nodes.
2. Change 2 with topic foo is created on node X, the index data is
   replicated to all nodes, but the replication for the Git data is
   still pending.
3. The SubmittedTogether REST endpoint is invoked for change 1 on node
   Y. A query to the change index is executed to find all changes that
   have the same topic as change 1. The index query by topic returns
   change 1 and change 2 (because the index data is already replicated).
   Checking read permissions for change 2 fails because loading the
   change notes from Git fails (because the Git data was not replicated
   yet).

Instead of failing we now create the change notes from the index data so
that the read permission check can be performed successfully.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I4c3284877483b8248f8c2a1f7f529d93f41dbbb8