- - stop trying to calculate an upper bound on the index file size.
- Instead, just mmap it at initial size + delta, and periodically
- re-mmap it during index building if it grows too big. If we run
- out of address space, we'll hear about it eventually; and
- computing upper bounds given the new optimised index tends to be
- a factor of five out, which is bad because it'll lead to running
- out of theoretical address space and erroneously reporting
- failure long before we run out of it for real.
-