object-file.c: use size_t for header lengths

object-file.c: use size_t for header lengths

Continue walking the code path for the >4GB `hash-object --literally`
test. The `hash_object_file_literally()` function internally uses both
`hash_object_file()` and `write_object_file_prepare()`. Both function
signatures use `unsigned long` rather than `size_t` for the mem buffer
sizes. Use `size_t` instead, for LLP64 compatibility.

While at it, convert those function's object's header buffer length to
`size_t` for consistency. The value is already upcast to `uintmax_t` for
print format compatibility.

Note: The hash-object test still does not pass. A subsequent commit
continues to walk the call tree's lower level hash functions to identify
further fixes.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>