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>