convert: make it safer to add conversion attributes

convert: make it safer to add conversion attributes

The places that need to pass an array of "struct git_attr_check" needed to
be careful to pass a large enough array and know what index each element
lied.  Make it safer and easier to code these.

Besides, the hard-coded sequence of initializing various attributes was
too ugly after we gained more than a few attributes.

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