[RFC] Fourth 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 handled inside git-checkout.

git-checkout will now also checkout/fetch any submodule
than is present (i.e., the corresponding subdirectory has a .git)
irrespective of the --submodules option.

A forced checkout of the submodule directory, i.e.,

git checkout submodule

will (clone and) checkout the submodule.

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

skimo