t5510: do not leave changed cwd

t5510 carefully keeps the cwd at the test root by using either subshells
or explicit cd'ing back to the root. Use a subshell for the last
subtest, too.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>