[REQ] Adding diff-merge capability into openapi-generator-cli generate

I was using openapi-generator-cli to create server stubs for my openapi 3.0.0 template. Realized that everytime I make a modification to schema and generate the server stubs for nodejs-express-server, I only have an option of

1. dry-run - mock run the stub generation and see which files are going to be impacted OR
2. override - simply overwrite the newly generated files

I would like to go a step further and trigger a diff-merge while doing generate.
As an alternative, right now, I can create stubs afresh on a new folder and merge from there to the original work-in-progress project folder.

However, Integration with diff/merge tools can be a great time and effort saver for repeated codegens. Most of the times, I go back and forth while creating the API template.

Start with the openapi-generator-cli generate flow and its existing dry-run and override options, using nodejs-express-server as the example generator. Determine how the command reports affected files and define the expected diff-merge behavior and tool integration; done means repeated generation can merge changes into an existing project without simply overwriting files.