Linus Torvalds [off-list ref] writes:
My personal suspicion is that we'll want to have a 64-bit index file some
day, and THAT is worthy of a format change. That day is not now, btw. It's
probably not even very close. Even the mozilla repo that was pushing the
limit was only doing so until it was optimized better, and now it's
apparently nowhere _near_ that limit.
But even then, we might well want to update _just_ the index file format.
We've tried this already, and I shelved the patch for 64-index
for now due to exactly the same reasoning as yours (and it would
have conflicted heavily with Shawn's windowed-mmap() patch). It
involved updating just the index file format, so you are right
on both counts.
But you are always right anyway, so it may not be a news at all
;-).