t/perf/p2000: add perf test for git-rm

The `p2000` tests demonstrate a ~96% execution time reduction for
'git rm' using a sparse index.

Test                                     before  after
-------------------------------------------------------------
2000.74: git rm -f f2/f4/a (full-v3)     0.66    0.88 +33.0%
2000.75: git rm -f f2/f4/a (full-v4)     0.67    0.75 +12.0%
2000.76: git rm -f f2/f4/a (sparse-v3)   1.99    0.08 -96.0%
2000.77: git rm -f f2/f4/a (sparse-v4)   2.06    0.07 -96.6%

Signed-off-by: Shaoxuan Yuan <[email protected]>

Author: ffyuanda

t/perf/p2000-sparse-operations.sh

@@ -123,5 +123,6 @@ test_perf_on_all git blame $SPARSE_CONE/f3/a
 test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
+test_perf_on_all git rm -f $SPARSE_CONE/a
 
 test_done