The git_mmap implementation was broken for file sizes that wouldn't fit
into a size_t (32 bits).  This was caused by intermediate variables that
were only 32 bits wide when they should be 64 bits.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>