Increase packedGit{Limit,WindowSize} on 64 bit systems.

Increase packedGit{Limit,WindowSize} on 64 bit systems.

If we have a 64 bit address space we can easily afford to commit
a larger amount of virtual address space to pack file access.
So on these platforms we should increase the default settings of
core.packedGit{Limit,WindowSize} to something that will better
handle very large projects.

Thanks to Andy Whitcroft for pointing out that we can safely
increase these defaults on such systems.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>