Fix new-workdir (again) to work on bare repositories

Fix new-workdir (again) to work on bare repositories

My day-job workflow involves using multiple workdirs attached to a
bunch of bare repositories.  Such repositories are stored inside of
a directory called "foo.git", which means `git rev-parse --git-dir`
will return "." and not ".git".  Under such conditions new-workdir
was getting confused about where the Git repository it was supplied
is actually located.

If we get "." for the result of --git-dir query it means we should
use the user supplied path as-is, and not attempt to perform any
magic on it, as the path is directly to the repository.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>