Introduce diff_filespec_is_binary()

Introduce diff_filespec_is_binary()

This replaces an explicit initialization of filespec->is_binary
field used for rename/break followed by direct access to that
field with a wrapper function that lazily iniaitlizes and
accesses the field.  We would add more attribute accesses for
the use of diff routines, and it would be better to make this
abstraction earlier.

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