Initial support for cloning submodules

From: Sven Verdoolaege <redacted>

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

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

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

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

will use the second.

The submodules are currently not checked out.

skimo