t5510: prefer "git -C" to subshell for followRemoteHEAD tests

t5510: prefer "git -C" to subshell for followRemoteHEAD tests

These tests set config within a sub-repo using (cd two && git config),
and then a separate test_when_finished outside the subshell to clean it
up. We can't use test_config to do this, because the cleanup command it
registers inside the subshell would be lost. Nor can we do it before
entering the subshell, because the config has to be set after some other
commands are run.

Let's switch these tests to use "git -C" for each command instead of a
subshell. That lets us use test_config (with -C also) at the appropriate
part of the test. And we no longer need the manual cleanup command.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>