Make AutoCRLF ternary variable.

This allows you to do:

[core]
AutoCRLF = input

and it should do only the CRLF->LF translation (ie it simplifies CRLF only
when reading working tree files, but when checking out files, it leaves
the LF alone, and doesn't turn it into a CRLF).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>