diff-tree: integrate with sparse index

The index is read in 'cmd_diff_tree' at two points:

1. The first index read was added in fd66bcc31ff (diff-tree: read the
index so attribute checks work in bare repositories, 2017-12-06) to deal
with reading '.gitattributes' content. 77efbb366ab (attr: be careful
about sparse directories, 2021-09-08) established that, in a sparse
index, we do _not_ try to load a '.gitattributes' file from within a
sparse directory.

2. The second index access point is involved in rename detection,
specifically when reading from stdin.This was initially added in
f0c6b2a2fd9 ([PATCH] Optimize diff-tree -[CM]--stdin, 2005-05-27), where
'setup' was set to 'DIFF_SETUP_USE_SIZE_CACHE |DIFF_SETUP_USE_CACHE'.
That assignment was later modified to drop the'DIFF_SETUP_USE_CACHE' in
ff7fe37b053 (diff.c: move read_index() code back to the caller,
2018-08-13).However, 'DIFF_SETUP_USE_SIZE_CACHE' seems to be unused as
of 6e0b8ed6d35 (diff.c: do not use a separate "size cache"., 2007-05-07)
and nothing about 'detect_rename' otherwise indicates index usage.

Hence we can just set the requires-full-index to false for "diff-tree".

Add tests that verify that 'git diff-tree' behaves correctly when the
sparse index is enabled and test to ensure the index is not expanded.

The `p2000` tests demonstrate a ~98% execution time reduction for
'git diff-tree' using a sparse index:

Test                                                before  after
-----------------------------------------------------------------------
2000.94: git diff-tree HEAD (full-v3)                0.05   0.04 -20.0%
2000.95: git diff-tree HEAD (full-v4)                0.06   0.05 -16.7%
2000.96: git diff-tree HEAD (sparse-v3)              0.59   0.01 -98.3%
2000.97: git diff-tree HEAD (sparse-v4)              0.61   0.01 -98.4%
2000.98: git diff-tree HEAD -- f2/f4/a (full-v3)     0.05   0.05 +0.0%
2000.99: git diff-tree HEAD -- f2/f4/a (full-v4)     0.05   0.04 -20.0%
2000.100: git diff-tree HEAD -- f2/f4/a (sparse-v3)  0.58   0.01 -98.3%
2000.101: git diff-tree HEAD -- f2/f4/a (sparse-v4)  0.55   0.01 -98.2%

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

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