Our status quo for octopus merges is not to compute the diff, regardless
if the parentNum parameter is set or not. This used to return the
COMMIT_MSG and MERGE_LIST entries only.
This change adds support for diffing against a specifc parent for
octopus merge commits. If no specific parent is specified, we do not
compute the auto-merge commit and still follow the old behavior.
This change is important to avoid merging changes with potentially
unreviewed files, since they did not appear in the Gerrit UI. We still
can follow up on this change by throwing an exception with a meaningful
message if the auto-merge is requested for an octopus merge, so that the
Gerrit UI can display that this operation is not supported.
Change-Id: I6670585c7860719a4c226aa054ca635f67d90106
27ce6452a9