Define 'crlf' attribute.

Define 'crlf' attribute.

This defines the semantics of 'crlf' attribute as an example.
When a path has this attribute unset (i.e. '!crlf'), autocrlf
line-end conversion is not applied.

Eventually we would want to let users to build a pipeline of
processing to munge blob data to filesystem format (and in the
other direction) based on combination of attributes, and at that
point the mechanism in convert_to_{git,working_tree}() that
looks at 'crlf' attribute needs to be enhanced.  Perhaps the
existing 'crlf' would become the first step in the input chain,
and the last step in the output chain.

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