rm: integrate with sparse-index

rm: integrate with sparse-index

Enable the sparse index within the `git-rm` command.

The `p2000` tests demonstrate a ~92% execution time reduction for
'git rm' using a sparse index.

Test                              HEAD~1            HEAD
--------------------------------------------------------------------------
2000.74: git rm ... (full-v3)     0.41(0.37+0.05)   0.43(0.36+0.07) +4.9%
2000.75: git rm ... (full-v4)     0.38(0.34+0.05)   0.39(0.35+0.05) +2.6%
2000.76: git rm ... (sparse-v3)   0.57(0.56+0.01)   0.05(0.05+0.00) -91.2%
2000.77: git rm ... (sparse-v4)   0.57(0.55+0.02)   0.03(0.03+0.00) -94.7%

----
Also, normalize a behavioral difference of `git-rm` under sparse-index.
See related discussion [1].

`git-rm` a sparse-directory entry within a sparse-index enabled repo
behaves differently from a sparse directory within a sparse-checkout
enabled repo.

For example, in a sparse-index repo, where 'folder1' is a
sparse-directory entry, `git rm -r --sparse folder1` provides this:

        rm 'folder1/'

Whereas in a sparse-checkout repo *without* sparse-index, doing so
provides this:

        rm 'folder1/0/0/0'
        rm 'folder1/0/1'
        rm 'folder1/a'

Because `git rm` a sparse-directory entry does not need to expand the
index, therefore we should accept the current behavior, which is faster
than "expand the sparse-directory entry to match the sparse-checkout
situation".

Modify a previous test so such difference is not considered as an error.

[1] https://github.com/ffyuanda/git/pull/6#discussion_r934861398

Helped-by: Victoria Dye <vdye@github.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>