In the later you can also forget project A exist. git log works in subdirectories out of the box and if you didn't touch any files in project A a rebase will be

In the later you can also forget project A exist. git log works in subdirectories out of the box and if you didn't touch any files in project A a rebase will be trivial without any merge conflicts. Branching is also free (unless you are still in the SVN stoneage that requires a copy of each file) so it doesn't matter if you branch the whole monorepo or just a single project.