t1092: expand repository data shape

t1092: expand repository data shape

As more features integrate with the sparse-index feature, more and more
special cases arise that require different data shapes within the tree
structure of the repository in order to demonstrate those cases.

Add several interesting special cases all at once instead of sprinkling
them across several commits. The interesting cases being added here are:

* Add sparse-directory entries on both sides of directories within the
  sparse-checkout definition.

* Add directories outside the sparse-checkout definition who have only
  one entry and are the first entry of a directory with multiple
  entries.

* Add filenames adjacent to a sparse directory entry that sort before
  and after the trailing slash.

Later tests will take advantage of these shapes, but they also deepen
the tests that already exist.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>