This series mostly looks good to me. There were a few parts I had
trouble following or convincing myself were safe, so there could be some
improvements with comments or more explicit commit messages, but no
problems apart from that.

Thanks!

For the rest who are following along, I somehow couldn't "git am" these
on latest master, but I could "git am" these on an old master commit
(from the time of the original work). The subsequent rebase works with
trivially-resolved merge conflicts except for one due to a21781011f
("index-pack: downgrade twice-resolved REF_DELTA to die()", 2020-02-04)
- but that was fixed by changing the relevant assert() command to
if/die().