win32: allocate buffer of maximum length for GetVolumeInformation()
It's documented that "the maximum buffer size is MAX_PATH+1", which is
slightly larger than 256.
https://msdn.microsoft.com/en-us/library/windows/desktop/aa364993(v=vs.85).aspx
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t