t4304: 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.

Tests are modified to assume correct (default) configuration at entry,
and to reset the modified configuration variables at the end.

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

t/t3404-rebase-interactive.sh

@@ -937,8 +937,7 @@ test_expect_success 'rebase --edit-todo can be used to modify todo' '
 test_expect_success 'rebase -i respects core.commentchar' '
 	git reset --hard &&
 	git checkout E^0 &&
-	git config core.commentchar "\\" &&
-	test_when_finished "git config --unset core.commentchar" &&
+	test_config core.commentchar "\\" &&
 	write_script remove-all-but-first.sh <<-\EOF &&
 	sed -e "2,\$s/^/\\\\/" "$1" >"$1.tmp" &&
 	mv "$1.tmp" "$1"