Copy updates as 1st-class citizens

The scaffolding landscape in Python currently is pretty bad, thanks for bringing to life this nice project.

Handling updates is pretty bad still. Imagine:

1. You have a scaffolding.
2. You copy it and start a new project.
3. Your new project evolves in a separate git repo.
4. You update your scaffolding.
5. You want to apply your scaffolding to your copied project again, but don’t want the default answers; you want the ones you used last time.
6. You don’t want the updates to touch the files that you already touched, unless they’ve been touched also in those lines in the source scaffolding.

cruft handles this pretty well, as it stores all answers in a json file, which also happens to include the git commit of the source when it was copied, and this allows it to extract the diff since that commit to the latest and apply only that diff to the destination.

How should copier copy (lol) this behavior?

First, when copying:

1. When a scaffolding is copied, store all answers in a file. It could be something like .copied.yml.
2. Store in .copied.yml also the source of the copy (e.g. _copied_from: gh:my/scaffolding).
3. When the scaffolding was a git repo, store also the copied commit. It could be a hidden key in .copied.yml (e.g.: _copied_commit: o3ho4ho5hoho2hoh2o).

Then, support running copier without arguments, which:

1. Fails if .copied.yml is absent.
2. Fails if _copied_from is absent in that file.
3. If _copied_commit is absent, it should be the equivalent of copier $_copied_from ..
4. If _copied_commit is present, it should be the equivalent of copied --from-commit $_copied_commit --to-commit HEAD $_copied_from ..

As you probably have guessed, we need to add new flags --from-commit and --to-commit (which defaults to HEAD), which:

1. Fails if source is not a git repo.
2. If present, will generate a patch from --from-commit to --to-commit.
3. It will only apply that diff.
4. Does nothing if both flags are equal.

Why all of this?

1. We need to be able to just execute copier in a pre-copied scaffolding and let it get only the diff since it was copied last time.
2. We need to be able to ensure a project is updated with its scaffolding, i.e. to raise an error in CI when that’s not the case.

[pykong]

@Yajo. The approach seems sane to me. The --commit flag as well. I am not yet sure about the `–no-diff`` flag.

Maybe put --no-diff into a separate PR.

Otherwise feel free to go ahead. Thanks for the great work! I really appreciate it.

btw - do you use https://gitter.im/ ?

[yajo]

Cool, I have a PR in the oven ready to improve this. I think tomorrow I’ll have time to push it.