marge-bot: merge MRs in order they are assigned to Marge-bot
The current behaviour means that every time someone posts a comment on the MR, for instance asking why Marge hasn't picked up the MR yet [1], they send their MR back at the end of the merge queue.
This is especially annoying on days where the Marge queue never goes down to zero, like it did for the last couple of days.
Let's use the IMO fairer assigned_at order.