p2000: add performance test for 'git add -p'

The previous two changes contributed performance improvements to 'git
apply' and 'git add -p' when using a sparse index. Add a performance
test to demonstrate this (and to help validate that performance remains
good in the future).

In the truncated test output below, we see that the full checkout
performance changes within noise expectations, but the sparse index
cases improve 33% and then 96%.

                      HEAD~3     HEAD~2         HEAD~1
---------------------------------------------------------
2000.118: (full-v3)     0.80   0.84 +5.0%     0.84  +5.0%
2000.119: (full-v4)     0.76   0.79 +3.9%     0.80  +5.3%
2000.120: (sparse-v3)   2.09   1.39 -33.5%    0.07 -96.7%
2000.121: (sparse-v4)   2.09   1.39 -33.5%    0.07 -96.7%

It is worth noting that if our test was more involved and had multiple
hunks to evaluate, then the time spent in 'git apply' would dominate due
to multiple index loads and writes. As it stands, we need the sparse
index improvement in 'git add -p' itself to confirm this performance
improvement.

Since the change for 'git add -i' is identical, we avoid a second test
case for that similar operation.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee

t/perf/p2000-sparse-operations.sh

@@ -135,5 +135,6 @@ test_perf_on_all git diff-tree HEAD
 test_perf_on_all git diff-tree HEAD -- $SPARSE_CONE/a
 test_perf_on_all "git worktree add ../temp && git worktree remove ../temp"
 test_perf_on_all git check-attr -a -- $SPARSE_CONE/a
+test_perf_on_all 'echo >>a && test_write_lines y | git add -p'
 
 test_done