Merge branch 'mc/clean-smudge-with-llp64'

Merge branch 'mc/clean-smudge-with-llp64'

The clean/smudge conversion code path has been prepared to better
work on platforms where ulong is narrower than size_t.

* mc/clean-smudge-with-llp64:
  clean/smudge: allow clean filters to process extremely large files
  odb: guard against data loss checking out a huge file
  git-compat-util: introduce more size_t helpers
  odb: teach read_blob_entry to use size_t
  t1051: introduce a smudge filter test for extremely large files
  test-lib: add prerequisite for 64-bit platforms
  test-tool genzeros: generate large amounts of data more efficiently
  test-genzeros: allow more than 2G zeros in Windows