Git allows joining unrelated repositories via remotes which, in turn, allows moving files and change history between them.

Some cases when this might be needed:

• Extract part of a big repository into a separate repository, preserving change history
• Splitting big repository to a set of smaller repositories
• Merge smaller repository into a bigger one (merge in library from the external repository)

Preserving the history has an important effect: after we do the change, for example, extract a part of a bigger repository into a separate repository, we can continue moving changes between them (merge updates from the big repository to the small one and back).

Below, I am assuming the first case (extract part of the big repository), other cases can be implemented with similar technique.

Setup

Assuming we have a source (upstream) repository and we want to extract a top-level /lib folder into a separate repository, first, create a destination repository:

mkdir dest-repo
cd dest-repo
git init

Add a main repository as remote and checkout master branch:

git remote add -f upstream [email protected]:myaccount/main.git
git checkout -b upstream_master upstream/master
git subtree split --prefix=lib/ -b upstream_lib

What we do above is: add the source (upstream) repository as a remote, checkout its master branch to the destination repository and then use git subtree split to create upstream_master_lib branch containing only /lib folder with all the change history.

Now we can merge the upstream files and history from the /lib folder to the target master branch:

git checkout master
git merge upstream_lib
git push origin HEAD

Update from upstream

Pull recent upstream repository master branch and re-split it to a new branch:

git checkout upstream_master
git pull
git subtree split --prefix=lib/ -b upstream_lib_YYYY.MM.DD

Merge the update:

git checkout master
git merge upstream_lib_YYYY.MM.DD
git push origin HEAD