Merge branch 'cb/parse-magnitude'

Move machinery to parse human-readable scaled numbers like 1k, 4M,
and 2G as an option parameter's value from pack-objects to
parse-options API, to make it available to other codepaths.

* cb/parse-magnitude:
  parse-options: move unsigned long option parsing out of pack-objects.c
  test-parse-options: update to handle negative ints