Fix 'crlf' attribute semantics.

Fix 'crlf' attribute semantics.

Earlier we said 'crlf lets the path go through core.autocrlf
process while !crlf disables it altogether'.  This fixes the
semantics to:

 - Lack of 'crlf' attribute makes core.autocrlf to apply
   (i.e. we guess based on the contents and if platform
   expresses its desire to have CRLF line endings via
   core.autocrlf, we do so).

 - Setting 'crlf' attribute to true forces CRLF line endings in
   working tree files, even if blob does not look like text
   (e.g. contains NUL or other bytes we consider binary).

 - Setting 'crlf' attribute to false disables conversion.

Signed-off-by: Junio C Hamano <junkio@cox.net>