If submodules exists for a git repo then checkout their index as well
into the appropriate path.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

A 'head' in a formula can now specify which download strategy to use via
a ':using' specification:

    head 'http://svn.macosforge.org/repository/darwinbuild/trunk/',
        :using => :svn

This reduces the number of cases where "download_strategy" needs to be
overriden.