It's only a preparatory step, as checking whether a size_t is less than SIZE_MAX is moot. In a follow-up commit, however, the `filesize' field of the struct got_pack will become off_t and these checks will kick in. This also makes consistent how we guard mmap(2) against empty files. ok and improvements stsp@

Prepares for sharing code between got-index-pack and future gotd(8).

"looks good to me" millert@

This approach uses more memory but is much faster. To offset the additional memory usage somewhat the cache now stores very small deltas only. However, overall memory usage goes up. Hopefully we will find a way to reduce this later. ok op@

The singly-linked tail queue macros were added to OpenBSD 6.9 and are more widely available on other systems. ok stsp

Same change as 16aeacf7088d, for subdirectories other than lib/