fixup! t1092: test interesting sparse-checkout scenarios

Making the 'blame with pathspec outside sparse definition' test more robust.
Previously, this test just looked for a generic "failure" and didn't verify
that sparse index and non-sparse index provide the same error. We now fail only
when the error messages from sparse index and non-sparse index don't match (and
still document the behavior in the test).

t/t1092-sparse-checkout-compatibility.sh

@@ -486,15 +486,14 @@ test_expect_success 'blame with pathspec inside sparse definition' '
 	test_all_match git blame deep/deeper1/deepest/a
 '
 
-# TODO: blame currently does not support blaming files outside of the
-# sparse definition. It complains that the file doesn't exist locally.
-test_expect_failure 'blame with pathspec outside sparse definition' '
+test_expect_success 'blame with pathspec outside sparse definition' '
 	init_repos &&
 
-	test_all_match git blame folder1/a &&
-	test_all_match git blame folder2/a &&
-	test_all_match git blame deep/deeper2/a &&
-	test_all_match git blame deep/deeper2/deepest/a
+	test_sparse_match git sparse-checkout set &&
+	test_sparse_match test_must_fail git blame folder1/a &&
+	test_sparse_match test_must_fail git blame folder2/a &&
+	test_sparse_match test_must_fail git blame deep/deeper2/a &&
+	test_sparse_match test_must_fail git blame deep/deeper2/deepest/a
 '
 
 # TODO: This behaves correctly in microsoft/git. Why?