update-index: add tests for sparse-checkout compatibility

Introduce tests for a variety of `git update-index` use cases, including
performance scenarios. Tests are intended to exercise `update-index` with
options that change the commands interaction with the index (e.g.,
`--again`) and with files/directories inside and outside a sparse checkout
cone.

Of note is that these tests clearly establish the behavior of `git
update-index --add` with untracked, outside-of-cone files. Unlike `git add`,
which fails with an error when provided with such files, `update-index`
succeeds in adding them to the index. Additionally, the `skip-worktree` flag
is *not* automatically added to the new entry. Although this is pre-existing
behavior, there are a couple of reasons to avoid changing it in favor of
consistency with e.g. `git add`:

* `update-index` is low-level command for modifying the index; while it can
  perform operations similar to those of `add`, it traditionally has fewer
  "guardrails" preventing a user from doing something they may not want to
  do (in this case, adding an outside-of-cone, non-`skip-worktree` file to
  the index)
* `update-index` typically only exits with an error code if it is incapable
  of performing an operation (e.g., if an internal function call fails);
  adding a new file outside the sparse checkout definition is still a valid
  operation, albeit an inadvisable one
* `update-index` does not implicitly set flags (e.g., `skip-worktree`) when
  creating new index entries with `--add`; if flags need to be updated,
  options like `--[no-]skip-worktree` allow a user to intentionally set them

All this to say that, while there are valid reasons to consider changing the
treatment of outside-of-cone files in `update-index`, there are also
sufficient reasons for leaving it as-is.

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

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