Git_clone_include_submodule

git clone 指定分支，并 register and update submodules

If your submodule was added in a branch be sure to include it in your clone command…

git clone -b <branch_name> --recursive <remote> <directory>

If --recursive is specified, this command will recurse into the registered submodules, and update any nested submodules within.

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参考： How to `git clone` including submodules?