Fix CDPATH problem.

Fix CDPATH problem.

CDPATH has two problems:

 * It takes scripts to unexpected places (somebody had
   CDPATH=..:../..:$HOME and the "cd" in git-clone.sh:get_repo_base
   took him to $HOME/.git when he said "clone foo bar" to clone a
   repository in "foo" which had "foo/.git").  CDPATH mechanism does
   not implicitly give "." at the beginning of CDPATH, which is
   the most irritating part.

 * The extra echo when it does its thing confuses scripts further.

Most of our scripts that use "cd" includes git-sh-setup so the problem
is primarily fixed there.  git-clone starts without a repository, and
it needs its own fix.

Signed-off-by: Junio C Hamano <junkio@cox.net>