>A disadvantage of git add -p is that it allows you to create a commit that never existed on the development system, and, hence, cannot have been tested.

The things you commit can always be different from what you test. In fact, if you know what you are doing, it is a feature to be able to create commits like this without testing. I certainly don't need to test my "Fixed typo in comments" commits...

>How do people handle that? One way would be to follow it up with git stash, running tests, and, if necessary, git amend, but that can get cumbersome soon.

Well, your CI system should be testing for you. But if you must test it locally, `git rebase` can either pause between steps or execute commands between steps. Finish making commits. If you made, say, 3 commits, then do `git rebase -i HEAD~3` and do break/exec as desired.