Delete diverged branches

Description

The automatic repo mirroring keeps the fork synchronized with the upstream repo. Since the latest changes, already merged branches from the upstream repo are not resynced after deletion.

But there are still a lot of branches marked with Diverged from upstream in the branch list.

It would be handy to delete them all by clicking Delete merged branchens button on the branch list.

Proposal

Delete those branches by click on the Delete merged branches button on the branch list

/cc @rdavila