git-clone: don't get fooled by $PWD

git-clone: don't get fooled by $PWD

If you have /home/me/git symlink pointing at /pub/git/mine,
trying to clone from /pub/git/his/ using relative path would not
work as expected:

	$ cd /home/me
        $ cd git
        $ ls ../
        his    mine
        $ git clone -l -s -n ../his/stuff.git

This is because "cd ../his/stuff.git" done inside git-clone to
check if the repository is local is confused by $PWD, which is
set to /home/me, and tries to go to /home/his/stuff.git which is
different from /pub/git/his/stuff.git.

We could probably say "set -P" (or "cd -P") instead, if we know
the shell is POSIX, but the way the patch is coded is probably
more portable.

[jc: this is updated with Andy Whitcroft's improvements]

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