Display relevant release notes after update

[bradbeattie]

When a developer uses my template and runs copier update, I'd like to be able to give them a summary of what they see when they git status. I can display a static message with message_after_update that links to my template's release notes, but there's a level of indirection there that's unsatisfying.

Some ideas, but I'm unclear on how acceptable each would be:

1. Just use hooks. I don't care for this approach as hooks come with warnings and security vulnerabilities and issues of trust.
2. A new section in copier.yml: Not terrible, but not as compartmentalized.
3. A capacity to read the template repo's CHANGELOG.md and display only the relevant sections: My favoured approach.

I haven't written a PR yet, but I'm happy to take on the effort of developing this. I just wanted to avoid writing it up if this idea doesn't fit within the goals of the project.

[sisp]

I see some benefit in printing the changelog after an update, but I also see some edge cases that may make this difficult to automate properly:

• What do you print when a user updates to an unreleased template version, e.g. using --vcs-ref <commit hash>?
• What do you print when a user updates to the same template version and only changes answers? You probably wouldn't show the changelog then.
• What do you print when a user downgrades to an older template version?

Also, there are a few different changelog formats, and template maintainers may even invent their own. If we supported this in Copier, we'd at least need to support a range of common changelog formats, perhaps offer a way to configure custom ones.

How about this: Migration tasks have access to a few additional variables like _version_from/_version_pep440_from and _version_to/_version_pep440_to. If we exposed them in _message_{before,after}_update, then you might be able to implement your own pure-Jinja changelog printing logic along the lines of this:

_message_after_update: |
  {%- if _version_pep440_from and _version_pep440_to and _version_pep440_to > _version_pep440_from %}
  {%- filter regex_search('(?ms)^## \[' ~ _version_to ~ '\].*?(?=^## |\Z)') %}
  {%- include 'CHANGELOG.md' -%}
  {%- endfilter %}
  {%- endif %}

[bradbeattie]

That'd do!