Fix auto errata pusher

I hope.

I think the problem was that it commited the update to the errata on a
detached HEAD, and then failed to transfer the commit when it rebased on
master.  Hence we now checkout master first, and then rebase (to make
absolutely sure we haven't missed anything), and then cherry-pick.

Also, make the marker script work even if there have been multiple merges since the erratum.