MinGW: 64-bit file offsets

MinGW: 64-bit file offsets

The type 'off_t' should be used everywhere so that the bit-depth of that
type can be adjusted in the standard C library, and you just need to
recompile your program to benefit from the extended precision.

Only that it was not done that way in the MS runtime library.

This patch reroutes off_t to off64_t and provides the other necessary
changes so that finally, clones larger than 2 gigabyte work on Windows
(provided you are on a file system that allows files larger than 2gb).

Initial patch by Sickboy <sb@dev-heaven.net>.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>