Set the active codepage for Windows syscalls

All syscalls performed by the library will assume this codepage when converting paths and strings to use by the Windows kernel.

The default value of UTF-8 will work automatically with most Git repositories created on Unix systems.

This settings needs only be changed when working with repositories that contain paths in specific, non-UTF codepages.

A full list of all available codepage identifiers may be found at:

http://msdn.microsoft.com/en-us/library/windows/desktop/dd317756(v=vs.85).aspx