Merge branch 'jk/4gb-idx'

The code was not prepared to deal with pack .idx file that is
larger than 4GB.

* jk/4gb-idx:
  packfile: detect overflow in .idx file size checks
  block-sha1: take a size_t length parameter
  fsck: correctly compute checksums on idx files larger than 4GB
  use size_t to store pack .idx byte offsets
  compute pack .idx byte offsets using size_t