clone: shallow-submodules should be single-branch by default

I noticed a couple places where the behavior of recursive clones for shallow submodules does not match what is implied by the documentation. Shallow submodules should be, but aren't, single branch by default. It would also be useful to allow users to override the shallow specification in gitmodules on the command line for clones as they can for submodule update. The modification here for the former is a bit ugly, but hopefully at least gets the point across to start a discussion. First time submitting a patch here, hopefully I'm getting the process right.

Bruce Perry (2):
clone: shallow-submodules should be single-branch by default
clone: no-shallow-submodules clone overrides option in gitmodules

 Documentation/git-clone.txt         |  3 ++
 Documentation/gitmodules.txt        |  4 +--
 builtin/clone.c                     | 10 ++++--
 t/t5614-clone-submodules-shallow.sh | 52 +++++++++++++++++++++++------
 4 files changed, 53 insertions(+), 16 deletions(-)

base-commit: daed0c68e94967bfbb3f87e15f7c9090dc1aa1e1
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1740%2Fbaperry2%2Fsubmods-clone-bug-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1740/baperry2/submods-clone-bug-v1
Pull-Request: https://github.com/git/git/pull/1740

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