Try to fix the auto-errata-pusher

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 cherry-pick after rebase.
Make the errata marker work after merges

The old code listed all the merges between the erratum and the current
head.  By inspection of what the errata marking command does for
39dfbd8 and what 39dfbd8 actually got marked as, we want the last sha1
in the list.