attr: "binary" attribute should choose built-in "binary" merge driver

attr: "binary" attribute should choose built-in "binary" merge driver

The built-in "binary" attribute macro expands to "-diff -text", so
that textual diff is not produced, and the contents will not go
through any CR/LF conversion ever.  During a merge, it should also
choose the "binary" low-level merge driver, but it didn't.

Make it expand to "-diff -merge -text".

Signed-off-by: Junio C Hamano <gitster@pobox.com>