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]