Move ChangeMessage out of transaction during cherry-pick

In the case of CherryPickChange, the ChangeMessage does not belong to
the same change, so it cannot be in a transaction against the same
root key. This applies to both the new patch set insertion and new
change creation code paths.

Change-Id: Iae76682a45ec327fef6af25422e459ffd4557e05
2 files changed