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.