t4108: use `test_config` instead of `git config`

Since `git config` leaves the configurations set even after the test
case completes, use `test_config` instead so that the configurations are
reset once the test case finishes.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4108-apply-threeway.sh

@@ -70,7 +70,7 @@ test_expect_success 'apply with --3way' '
 '
 
 test_expect_success 'apply with --3way with rerere enabled' '
-	git config rerere.enabled true &&
+	test_config rerere.enabled true &&
 
 	# Merging side should be similar to applying this patch
 	git diff ...side >P.diff &&