Marge force-pushes over last-second pushes from MR author

Everything's in the title, but in other words, the following scenario:

sequenceDiagram
    Author->>MR: pushes version 1
    Author->>MR: assigns Marge
    MR->>Marge: picks up MR, fetches & rebases it
    Author->>MR: pushes version 2
    Marge->>MR: pushes version 1 rebased

causes the last push from the author to be lost.

Marge needs to fail when it tries to push and the remote was updated under its feet.