t1091: use check_files to reduce boilerplate

t1091: use check_files to reduce boilerplate

When testing the sparse-checkout feature, we need to compare the
contents of the working-directory against some expected output.
Using here-docs was useful in the beginning, but became repetetive
as the test script grew.

Create a check_files helper to make the tests simpler and easier
to extend. It also reduces instances of bad here-doc whitespace.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>