Provide some guidance on updated scaffold features getting applied to already-created projects

[alexeagle]

A scaffold can gain features over time. Anyone who already stamped out the scaffold essentially got a fork of the commit history to the scaffold template.

Like version control, a user who created their project from the scaffold might want to "merge" some of those improvements to the project they've already created. Or, they might have answered "No" to a prompt the first time, but then when they run again they say "Yes".

Of course the --no-clobber flag isn't sufficient for this: it would create new files but has no way to add contents into existing ones.
And like version control, merge conflicts can happen and would need some affordance for resolving (most tools would do it nicely if we literally wrote merge conflict range markers into their file)

Starting a discussion since I'm not sure if there's already a good way to do this, or it's a feature that needs some design thought.