Sorry, my last email mentioned git subtree’s README but I was wrong.
Please refer to it’s git-subtree.txt  file instead.
An article mentioning git subtree  mentions that a downside is having to
explicitly split changes to a sub-project out of the parent project.
“The downside is that all of your subproject files are present in the
parent repository, which means you’re giving up some of the reason for
originally splitting up your project repositories: having one canonical
repository for a given set of shared code. If someone makes a change to a
subproject, they can merge it with other changes locally, but they’d have
to explicitly split that change back out of their project if they wanted to
share it with projects.”
In fact that point is also made in git-subtree.txt .
“If the standalone library gets updated, you can automatically merge the
changes into your project; if you update the library inside your project,
you can “split” the changes back out again and merge them back into the