implement fetching of moved submodules

implement fetching of moved submodules

We store the changed submodules paths to calculate which submodule needs
fetching. This does not work for moved submodules since their paths do
not stay the same in case of a moved submodules. In case of new
submodules we do not have a path in the current checkout, since they
just appeared in this fetch.

It is more general to collect the submodule names for changes instead of
their paths to include the above cases. If we do not have a
configuration for a gitlink we rely on constructing a default name from
the path if a git repository can be found at its path. We skip
non-configured gitlinks whose default name collides with a configured
one.

With the change described above we implement 'on-demand' fetching of
changes in moved submodules.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>