support/download/git: add support for submodules

support/download/git: add support for submodules

Some git repositories may be split into a master repository and
submodules. Up until now, we did not have support for submodules,
because we were using bare clones, in which it is not possible to
update the list of submodules.

Now that we are using plain clones with a working copy, we can retrieve
the submdoules.

Add an option to the git download helper to kick the update of
submodules, so that they are only fetched for those packages that
require them. Also document the existing -q option at the same time.

Submodules have a .git file at their root, which contains the path to
the real .git directory of the master repository. Since we remove it,
there is no point in keeping those .git files either.

Note: this is currently unused, but will be enabled with the follow-up
patch that adds the necessary parts in the pkg-generic and pkg-download
infrastructures.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Matt Weber <matt@thewebers.ws>
Reviewed-by: Matt Weber <matt@thewebers.ws>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>