clone: add `--shallow-submodules` flag

When creating a shallow clone of a repository with submodules, the depth
argument does not influence the submodules, i.e. the submodules are done
as non-shallow clones. It is unclear what the best default is for the
depth of submodules of a shallow clone, so we need to have the possibility
to do all kinds of combinations:

* shallow super project with shallow submodules
  e.g. build bots starting always from scratch. They want to transmit
  the least amount of network data as well as using the least amount
  of space on their hard drive.
* shallow super project with unshallow submodules
  e.g. The superproject is just there to track a collection of repositories
  and it is not important to have the relationship between the repositories
  intact. However the history of the individual submodules matter.
* unshallow super project with shallow submodules
  e.g. The superproject is the actual project and the submodule is a
  library which is rarely touched.

The new switch to select submodules to be shallow or unshallow supports
all of these three cases.

It is easy to transition from the first to the second case by just
unshallowing the submodules (`git submodule foreach git fetch
--unshallow`), but it is not possible to transition from the second to the
first case (as we would have already transmitted the non shallow over
the network). That is why we want to make the first case the default in
case of a shallow super project. This leads to the inconvenience in the
second case with the shallow super project and unshallow submodules,
as you need to pass `--no-shallow-submodules`.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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