In case you want to put project-a into a subdirectory, you can use git-filter-repo (filter-branch is discouraged)

If you want the project's history to look as though all files have always been in the directory foo/bar, then you need to do a little surgery. Use git filter-branch with the "tree filter" to rewrite the commits so that anywhere foo/bar doesn't exist, it is created and all files are moved to it: