checkout-index: add performance test for checkout-index --all

`git checkout-index --all` is a subcommand of `git stash`, so it is helpful
to verify the performance of that particular usage.

Signed-off-by: Victoria Dye <vdye@github.com>

t/perf/p2000-sparse-operations.sh

@@ -113,6 +113,7 @@ test_perf_on_all git checkout -f -
 test_perf_on_all git reset
 test_perf_on_all git reset --hard
 test_perf_on_all git reset -- does-not-exist
+test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove
 test_perf_on_all git diff
 test_perf_on_all git diff --staged