t6026-merge-attr: clean up background process at end of test case

The process spawned in the hook uses the test's trash directory as CWD.
As long as it is alive, the directory cannot be removed on Windows.
Although the test succeeds, the 'test_done' that follows produces an
error message and leaves the trash directory around. Kill the process
before the test case advances.
Helped-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t6026-merge-attr.sh

@@ -185,7 +185,9 @@ test_expect_success 'custom merge does not lock index' '
 	git reset --hard anchor &&
 	write_script sleep-one-second.sh <<-\EOF &&
 		sleep 1 &
+		echo $! >sleep.pid
 	EOF
+	test_when_finished "kill \$(cat sleep.pid)" &&
 
 	test_write_lines >.gitattributes \
 		"* merge=ours" "text merge=sleep-one-second" &&