t7610: don't use test_config in a subshell

test_config uses test_when_finished to reset the configuration after the
test, but this does not work inside a subshell.  This does not cause a
problem here because the first thing the next test does is to set this
config variable itself, but we are about to add a check that will
complain when test_when_finished is used in a subshell.

In this case, "subdir" not a submodule so test_config has the same
effect when run at the top level and can simply be moved out of the
subshell.
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7610-mergetool.sh

@@ -174,9 +174,9 @@ test_expect_success 'mergetool skips autoresolved' '
 '
 
 test_expect_success 'mergetool merges all from subdir' '
+	test_config rerere.enabled false &&
 	(
 		cd subdir &&
-		test_config rerere.enabled false &&
 		test_must_fail git merge master &&
 		( yes "r" | git mergetool ../submod ) &&
 		( yes "d" "d" | git mergetool --no-prompt ) &&