convert: make it harder to screw up adding a conversion attribute

convert: make it harder to screw up adding a conversion attribute

The current internal API requires the callers of setup_convert_check() to
supply the git_attr_check structures (hence they need to know how many to
allocate), but they grab the same set of attributes for given path.

Define a new convert_attrs() API that fills a higher level information that
the callers (convert_to_git and convert_to_working_tree) really want, and
move the common code to interact with the attributes system to it.

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