"git archive" learned to handle files that are larger than 8GB and
commits far in the future than expressible by the traditional US-TAR
format.
* jk/big-and-future-archive-tar:
archive-tar: drop return value
archive-tar: write extended headers for far-future mtime
archive-tar: write extended headers for file sizes >= 8GB
t5000: test tar files that overflow ustar headers
t9300: factor out portable "head -c" replacement