clone: the given repository dir should be relative to $PWD

the repository argument for git-clone should be relative to $PWD
instead of the given target directory.  The old behavior gave us
surprising success and you need a few minute to know why it worked.

GIT_DIR is already exported so no need to cd into $D. And this makes
$PWD for git-fetch-pack, which is the actual command to take the given
repository dir, the same as git-clone.

Signed-off-by: Yasushi SHOJI <yashi@atmark-techno.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>