how about using the "git subtree" to join repos?

[lhrkkk]

git subtree can join multi repos together and can push them back, is it a little better ?

[AdrianoDigital]

Depends on your use case. For importing libraries into your project git subtree is probably the better solution.

One downside of git subtree is that the history is not visible for the files under the "--prefix" sub-directory.

Example:
git init new-repo
cd new-repo
git commit --allow-empty -m "Intial, empty commit"
git subtree add --prefix=repo1 git@github.com:username/repo1.git master
git subtree add --prefix=repo2 git@github.com:username/repo2.git master

The imported history can be seen allright by
git log --oneline --all

However, if you try to look up the history of some specific file, you'll only see the subtree add commit:
git log --oneline --all repo1/
655d1004 Add 'repo1/' from commit '3ff71f86ddfee07c00021f82f078d01460a0a314'