status: use sparse-index throughout

By testing 'git -c core.fsmonitor= status -uno', we can check for the
simplest index operations that can be made sparse-aware. The necessary
implementation details are already integrated with sparse-checkout, so
modify command_requires_full_index to be zero for cmd_status().

In refresh_index(), we loop through the index entries to refresh their
stat() information. However, sparse directories have no stat()
information to populate. Ignore these entries.

This allows 'git status' to no longer expand a sparse index to a full
one. This is further tested by dropping the "-uno" option and adding an
untracked file into the worktree.

The performance test p2000-sparse-checkout-operations.sh demonstrates
these improvements:

Test                                  HEAD~1           HEAD
-----------------------------------------------------------------------------
2000.2: git status (full-index-v3)    0.31(0.30+0.05)  0.31(0.29+0.06) +0.0%
2000.3: git status (full-index-v4)    0.31(0.29+0.07)  0.34(0.30+0.08) +9.7%
2000.4: git status (sparse-index-v3)  2.35(2.28+0.10)  0.04(0.04+0.05) -98.3%
2000.5: git status (sparse-index-v4)  2.35(2.24+0.15)  0.05(0.04+0.06) -97.9%

Note that since HEAD~1 was expanding the sparse index by parsing trees,
it was artificially slower than the full index case. Thus, the 98%
improvement is misleading, and instead we should celebrate the 0.34s to
0.05s improvement of 85%. This is more indicative of the peformance
gains we are expecting by using a sparse index.

Note: we are dropping the assignment of core.fsmonitor here. This is not
necessary for the test script as we are not altering the config any
other way. Correct integration with FS Monitor will be validated in
later changes.

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

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