sparse-index: update command for expand/collapse test

In anticipation of `git reset --hard` being able to use the sparse index
without expanding it, replace the command in `sparse-index is expanded and
converted back` with `git reset -- folder1/a`. This command will need to
expand the index to work properly, even after integrating the rest of
`reset` with sparse index.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1092-sparse-checkout-compatibility.sh

@@ -634,11 +634,15 @@ test_expect_success 'submodule handling' '
 	grep "160000 commit $(git -C initial-repo rev-parse HEAD)	modules/sub" cache
 '
 
+# When working with a sparse index, some commands will need to expand the
+# index to operate properly. If those commands also write the index back
+# to disk, they need to convert the index to sparse before writing.
+# This test verifies that both of these events are logged in trace2 logs.
 test_expect_success 'sparse-index is expanded and converted back' '
 	init_repos &&
 
 	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
-		git -C sparse-index -c core.fsmonitor="" reset --hard &&
+		git -C sparse-index reset -- folder1/a &&
 	test_region index convert_to_sparse trace2.txt &&
 	test_region index ensure_full_index trace2.txt
 '