The old behaviour was to unilaterally default to the cwd is the work tree
when GIT_DIR was set, but GIT_WORK_TREE wasn't, no matter if we are inside
the GIT_DIR, or if GIT_DIR is actually something like ../../../.git.
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>