check-attr: integrate with sparse-index

check-attr: integrate with sparse-index

Set the requires-full-index to false for "check-attr".

Add a test to ensure that the index is not expanded whether the files
are outside or inside the sparse-checkout cone when the sparse index is
enabled.

The `p2000` tests demonstrate a ~63% execution time reduction for
'git check-attr' using a sparse index.

Test                                            before  after
-----------------------------------------------------------------------
2000.106: git check-attr -a f2/f4/a (full-v3)    0.05   0.05 +0.0%
2000.107: git check-attr -a f2/f4/a (full-v4)    0.05   0.05 +0.0%
2000.108: git check-attr -a f2/f4/a (sparse-v3)  0.04   0.02 -50.0%
2000.109: git check-attr -a f2/f4/a (sparse-v4)  0.04   0.01 -75.0%

Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>