tests(scalar): tighten the stale `scalar.repo` test some

As pointed out by Stolee, the previous incarnation of this test case was
not stringent enough: we want to verify that _only_ the stale entries
are removed (previously, the test case would have succeeded even if all
entries had been removed).

Let's rectify this and verify that the other entries are left intact.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t9210-scalar.sh

@@ -171,10 +171,13 @@ test_expect_success '`reconfigure -a` removes stale config entries' '
 	scalar register stale &&
 	scalar list >scalar.repos &&
 	grep stale scalar.repos &&
+
+	grep -v stale scalar.repos >expect &&
+
 	rm -rf stale &&
 	scalar reconfigure -a &&
 	scalar list >scalar.repos &&
-	! grep stale scalar.repos
+	test_cmp expect scalar.repos
 '
 
 test_expect_success 'scalar delete without enlistment shows a usage' '