New hardlink clone code in tip

[Matt Mackall]

I've applied Stephen Darnell's symlink patch to tip, after reworking
it for the new copyfile code. This gives smarter hardlink behavior with
fallbacks for filesystems that don't support hardlinking and most
notably, provides hardlinking support on Windows NTFS.

The Windows link code requires a mode from ActiveState Python, but
falls back to normal behavior if it's not available.

Windows and UNIX folks both: please test this out and let me know if
you have any problems.

-- 
Mathematics is the supreme nostalgia of our time.