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>

git-clone.sh

@@ -5,6 +5,9 @@
 # 
 # Clone a repository into a different directory that does not yet exist.
 
+# See git-sh-setup why.
+unset CDPATH
+
 usage() {
 	echo >&2 "* git clone [-l [-s]] [-q] [-u <upload-pack>] <repo> <dir>"
 	exit 1

git-sh-setup.sh

@@ -6,6 +6,12 @@
 : ${GIT_DIR=.git}
 : ${GIT_OBJECT_DIRECTORY="$GIT_DIR/objects"}
 
+# Having this variable in your environment would break scripts because
+# you would cause "cd" to be be taken to unexpected places.  If you
+# like CDPATH, define it for your interactive shell sessions without
+# exporting it.
+unset CDPATH
+
 die() {
 	echo "$@" >&2
 	exit 1