Merge branch 'bc/racy-4gb-files'

The index file has room only for lower 32-bit of the file size in the cached stat information, which means cached stat information will have 0 in its sd_size member for a file whose size is multiple of 4GiB. This is mistaken for a racily clean path. Avoid it by storing a bogus sd_size value instead for such files. * bc/racy-4gb-files: Prevent git from rehashing 4GiB files t: add a test helper to truncate files