t1413: use tar to save and restore entire .git directory

This makes the test independent of the particulars of the storage formats.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1413-reflog-detach.sh

@@ -7,8 +7,7 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 . ./test-lib.sh
 
 reset_state () {
-	git checkout main &&
-	cp saved_reflog .git/logs/HEAD
+	rm -rf .git && "$TAR" xf .git-saved.tar
 }
 
 test_expect_success setup '
@@ -17,7 +16,7 @@ test_expect_success setup '
 	git branch side &&
 	test_tick &&
 	git commit --allow-empty -m second &&
-	cat .git/logs/HEAD >saved_reflog
+	"$TAR" cf .git-saved.tar .git
 '
 
 test_expect_success baseline '