Support adding new pull requests to an existing forward port

[xmo-odoo]

Currently adding a new PR to an existing forward port sequence is not possible: the validation of the existing fw will trigger _port_forward which will immediately port the "true" forward ports to the next target.

After the batch gets merged, ForwardPortTasks._process_item runs, finds that at least one of the batch's PR was already forward ported, and bails on the entire thing (because already ported).

This can make it difficult to fix PRs which break some other things in their forward ports, or require further adaptations.