archive-tar: write extended headers for far-future mtime

The ustar format represents timestamps as seconds since the epoch, but only has room to store 11 octal digits. To express anything larger, we need to use an extended header. This is exactly the same case we fixed for the size field in the previous commit, and the solution here follows the same pattern. This is even mentioned as an issue in f2f0267 (archive-tar: use xsnprintf for trivial formatting, 2015-09-24), but since it only affected things far in the future, it wasn't deemed worth dealing with. But note that my calculations claiming thousands of years were off there; because our xsnprintf produces a NUL byte, we only have until the year 2242 to fix this. Given that this is just around the corner (geologically speaking, anyway), and because it's easy to fix, let's just make it work. Unlike the previous fix for "size", where we had to write an individual extended header for each file, we can write one global header (since we have only one mtime for the whole archive). There's a slight bit of trickiness there. We may already be writing a global header with a "comment" field for the commit sha1. So we need to write our new field into the same header. To do this, we push the decision of whether to write such a header down into write_global_extended_header(), which will now assemble the header as it sees fit, and will return early if we have nothing to write (in practice, we'll only have a large mtime if it comes from a commit, but this makes it also work if you set your system clock ahead such that time() returns a huge value). Note that we don't (and never did) handle negative timestamps (i.e., before 1970). This would probably not be too hard to support in the same way, but since git does not support negative timestamps at all, I didn't bother here. After writing the extended header, we munge the timestamp in the ustar headers to the maximum-allowable size. This is wrong, but it's the least-wrong thing we can provide to a tar implementation that doesn't understand pax headers (it's also what GNU tar does). Helped-by: René Scharfe <l.s.r@web.de> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>