How to move a repo as a sub-directory of another repo while preserving the commits

March 12, 2022 (updated in October 11, 2024)

To merge a Repo B into Repo A as a subfolder, run this command inside Repo A:

git subtree add -P $prefix $repo $rev

Set prefix to the name of the subdirectory, repo to the clone URL of Repo B, and rev to the revision of Repo B you want (HEAD if latest). This will take the history of Repo B and merge it with Repo A, along with an additional merge commit.