git-checkout.sh: handle running git-checkout from a different directory

git-checkout.sh: handle running git-checkout from a different directory

"$DIR" might not be a full path and it might not have `pwd` as ".."
directory. So use `cd -` to undo the first `cd` command.

Also, use `basename` to make a symbolic link with a relative path.

This doesn't matter yet but it will when for example the commands to
clone OVMF is been run from tools/ rather than tools/firmware/.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>