This has the effect of increasing the address space from 2GB to 4GB under
64-bit Windows, reducing the likelihood of an "out of memory" error when
e.g.  repacking a large repository.  The test suite passes with this
patch, with and without the MEM_TOP_DOWN flag added to all VirtualAlloc
calls.  While this is no guarantee that there are no issues with large
memory support (it could break Git on other setups than mine, for
example), it at least increases the chance that nothing obvious goes wrong
(such as errors introduced by faulty sign extension, say, with ssize_t).

[PT: Resolves github issue #12]
Signed-off-by: NPierre le Riche <github@pleasedontspam.me>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>