recursive cloning with --reference-if-able + --dissociate

[Rasmus Villemoes <rv@rasmusvillemoes.dk>]

At $dayjob, most of our projects are organized as a rather small
superproject, with a number of git submodules. Usually, one of those
submodules is a linux kernel git tree.

For CI testing, we create a fresh docker container which starts by doing
a "git clone --recursive $superproject", which takes quite a while due
to the linux repo. So we'd like to speed that up by having a reasonable
up-to-date linux repo on each build slave, mapping that into the docker
container, and then telling git "when you get to cloning the src/linux
submodule, look over here for objects", i.e. having some way to have
--reference-if-able and --disassociate in effect for submodules.

If this is already possible, I can't find it in the documentation. And
if it doesn't exist, I can't really think of a good way to define the UI
for giving those options (other than, perhaps, having a top-level
--reference-if-able pass down to the submodules with the meaning 'when
initializing the submodule at some/path, try to use the submodule, if
any, at some/path in the referenced top-level repo'".

Rasmus