t1092: add tests for 'git check-attr'

t1092: add tests for 'git check-attr'

Add tests for `git check-attr`, make sure attribute file does get read
from index when path is either inside or outside of sparse-checkout
definition.

Add a test named 'diff --check with pathspec outside sparse definition'.
It starts by disabling the trailing whitespace and space-before-tab
checks using the core. whitespace configuration option. Then, it
specifically re-enables the trailing whitespace check for a file located
in a sparse directory by adding a whitespace=trailing-space rule to the
.gitattributes file within that directory. Next, create and populate the
folder1 directory, and then add the .gitattributes file to the index.
Edit the contents of folder1/a, add it to the index, and proceed to
"re-sparsify" 'folder1/' with 'git sparse-checkout reapply'. Finally,
use 'git diff --check --cached' to compare the 'index vs. HEAD',
ensuring the correct application of the attribute rules even when the
file's path is outside the sparse-checkout definition.

Mark the two tests 'check-attr with pathspec outside sparse definition'
and 'diff --check with pathspec outside sparse definition' as
'test_expect_failure' to reflect an existing issue where the attributes
inside a sparse directory are ignored. Ensure that the 'check-attr'
command fails to read the required attributes to demonstrate this
expected failure.

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