如需将project-a的project-a-toBeMergedTag tag代码 merge到 project-b中：
cd path/to/project-b
git remote add project-a /path/to/project-a
//查看所添加的remote仓库
git remote -v
git fetch project-a --tags
//merge代码
git pull project-a project-a-toBeMergedTag
//git merge --allow-unrelated-histories project-a/master # or whichever branch you want to merge
git remote remove project-a
参考资料
[1] How do you merge two Git repositories?