submodules: don't stumble over symbolic links when cloning recursively

submodules: don't stumble over symbolic links when cloning recursively

Since 69c3051 (submodules: refactor computation of relative gitdir path)
cloning a submodule recursively fails for nested submodules when a
symbolic link is part of the path to the work tree of the superproject.

This happens when module_clone() tries to find the relative paths between
the work tree and the git dir. When a symbolic link in current $PWD points
to a directory that is at a different level, then determining the number
of "../" needed to traverse to the superproject's work tree leads to a
wrong result.

As there is no portable way to say "pwd -P", use cd_to_toplevel to remove
the link from $PWD, which fixes this problem.

A test for this issue has been added to t7406.

Reported-by: Bob Halley <halley@play-bow.org>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>