Official support for specifying --work-tree/GIT_WORK_TREE without
--git-dir/GIT_DIR was added with v1.7.4-rc3~2^2~2.  Update description
of GIT_WORK_TREE to reflect this.
Signed-off-by: NChris Rorvick <chris@rorvick.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>