If you intend to create a new repository for a gem that will only be used in one project,
consider creating this gem into the
vendor/gem directory in the parent project.
This will make shipping stuff easier. Now you don’t need to make the commit in the gem git
repo, then change the
Gemfile in the parent project, then bundle install, then commiting
Gemfile.lock in the parent repo.
Afterwards, if you really need it, you can split the entire git history of the gem by using
git subtree push:
git subtree push --prefix vendor/gem email@example.com/upstream-gem.git master