Update 'crlf' attribute semantics.

Update 'crlf' attribute semantics.

This updates the semantics of 'crlf' so that .gitattributes file
can say "this is text, even though it may look funny".

Setting the `crlf` attribute on a path is meant to mark the path
as a "text" file.  'core.autocrlf' conversion takes place
without guessing the content type by inspection.

Unsetting the `crlf` attribute on a path is meant to mark the
path as a "binary" file.  The path never goes through line
endings conversion upon checkin/checkout.

Unspecified `crlf` attribute tells git to apply the
`core.autocrlf` conversion when the file content looks like
text.

Setting the `crlf` attribut to string value "input" is similar
to setting the attribute to `true`, but also forces git to act
as if `core.autocrlf` is set to `input` for the path.

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