最后我找到了解决办法

git clone https://git..../A.git
cd A
for i in $(git branch -r | sed "s/.*origin\///"); do git branch -t $i origin/$i; done
git filter-branch --prune-empty --subdirectory-filter C -- --all
git filter-branch --prune-empty --tree-filter 'rm -rf C' -- --all
git remote -v
git remote set-url origin https://git.../C.git
git remote -v
git push -u origin --all

字符串
//对所有分支执行以下操作

git clone https://git..../A.git
cd A

型
删除文件夹c并添加子模块参考

gi add .
git commit -m "folder removed"
git push origin trunk 
git submodule add https://git..../C.git
cd C
git checkout trunk
git add .
git commit -m "new submodule ref added"
git push origin trunk

型