[RFC] Third round of support for cloning submodules

[skimo@liacs.nl]

From: Sven Verdoolaege <skimo@kotnet.org>

This patch series implements a mechanism for cloning submodules.
Each submodule is specified by a 'submodule.<submodule>.url'
configuration option, e.g.,

bash-3.00$ ./git-config --remote=http://www.liacs.nl/~sverdool/isa.git --get-regexp 'submodule\..*\.url' 
submodule.cloog.url /home/sverdool/public_html/cloog.git
submodule.cloog.url http://www.liacs.nl/~sverdool/cloog.git

git-checkout will use the first url that works.
E.g., a

git clone --submodules ssh://liacs/~/public_html/isa.git

followed by

git checkout origin/submodule

(which only works for me), will use the first url, while a

git clone --submodules http://www.liacs.nl/~sverdool/isa.git

followed by

git checkout origin/submodule

will use the second.

The cloning of submodules is now handled inside git-checkout.

I currently do not fetch after the initial clone, since
I'm not sure what ref to use for the revision I need to
fetch for the supermodule.
Suggestions are welcome.

Note that this is still WIP, so there is no need to remind
me that I still need to write documentation and tests.

skimo