The merge driver code was added in late 2015. I think
'406a654b::6f045b56 & user("sid0")' is a reasonable revset for finding
the relevant commits, including preparation for it. The code is very
poorly tested, which makes it very hard to maintain. It seems it's
only used by FB and they don't use this code base anymore, so let's
remove the code to make it easier for us to maintain our product.