Cherry-pick bot chooses the wrong parent in relation chains where the immediate parent is merged

Cherry-pick bot examines relation chains in order to maintain parentage across cherry-picks, but in the case where a cherry-pick is delayed for whatever reason long enough that it's parent is merged, cherry-pick bot will choose the original patchset of the now-merged parent instead of the merged sha. This is often harmless, but in the event that gerrit merges related changes in an integration out-of-order (impossible to predict), this can cause unexpected merge conflicts in the downstream cherry-pick.

To work around this, pickbot should pick on top of branch HEAD if its parent is in the MERGED state to avoid these types of conflicts.