subdirectory tests: code cleanup, uncomment test

subdirectory tests: code cleanup, uncomment test

Back when these tests were written, we wanted to make sure that Git
notices it is in a bare repository and "git show -s HEAD" would
refrain from complaining that HEAD might mean a file it sees in its
current working directory (because it does not).  But the version of
Git back then didn't behave well, without (doubly) being told that
it is inside a bare repository by exporting "GIT_DIR=.".  The form
of the test we originally wanted to have was left commented out as
a reminder.

Nowadays the test as originally intended works, so add it to the
test suite.  We'll keep the old test that explicitly sets GIT_DIR=.
to make sure that use case will not regress.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>