new-workdir: handle rev-parse --git-dir not always giving full path

new-workdir: handle rev-parse --git-dir not always giving full path

rev-parse --git-dir outputs a full path - except for the single case
of when the path would be $(pwd)/.git, in which case it outputs simply
.git.  Check for this special case and handle it.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>