Jeff King [off-list ref] writes:

Interestingly, I would have thought that the first update-index call
would "de-racify" the entry by rewriting the index. But we don't
actually write, presumably because we eventually realize that there are
no entries to update. But it might actually be worth doing the write,
because it avoids further file-content reads later on (and most
workflows tend to do a lot of reads; every git-status is going to rehash
the file until the next index update).

Yeah, there is a tradeoff of time being spent on writing the index
(which could be large) and having to rehash the content (which could
also have to happen number of times until the next index writeout),
and in hindsight I suspect that I got the tradeoff wrong when we did
the racy-git-avoidance thing.