CommitAPI:
https://gerrit-review.googlesource.com/Documentation/rest-api-projects.html#cherry-pick-commit
When the user tries to cherry pick a change onto an already
existing change in a different branch with CommitAPI (Change-ID is
provided in the commit message,
and the change with this Change-ID exists in the destination branch), it
results in NPE.
Instead, this operation should just create a new patch set on the
existing change.
When using CommitAPI, the source commit is not associated with the
change, but the existing destination change might have cherryPickOf
already set. Reset cherryPickOf in NoteDb for this case.
Change-Id: I11beb7de9be4aa60a3c5e6a045b46156d9b310da
b9608bef91