dir.c: Avoid c99 array initialization

dir.c: Avoid c99 array initialization

The following syntax:

        char foo[] = {
                [0] = 1,
                [7] = 2,
                [15] = 3
        };

is a c99 construct which some compilers do not support even though they
support other c99 constructs. This construct can be avoided by folding
these 'special' test cases into the sane_ctype array and making use of
the related infrastructure.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>