t3400: use test_config to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3400-rebase.sh

@@ -138,8 +138,7 @@ test_expect_success 'rebase a single mode change' '
 '
 
 test_expect_success 'rebase is not broken by diff.renames' '
-	git config diff.renames copies &&
-	test_when_finished "git config --unset diff.renames" &&
+	test_config diff.renames copies &&
 	git checkout filemove &&
 	GIT_TRACE=1 git rebase force-3way
 '