repo-create: allow creation from a GitHub template repository

[salewski]

Background reading

• https://github.blog/2019-06-06-generate-new-repositories-with-repository-templates/
• https://developer.github.com/v3/repos/#create-a-repository-using-a-template
• https://docs.github.com/en/free-pro-team@latest/rest/reference/repos#create-a-repository-using-a-template

GitHub v3 API endpoint:

    POST /repos/:template_owner/:template_repo/generate

Feature description

The ads-github-repo-create program could support a --from-tempalte=TMPL_REPO_NAME option to allow for the creation of a new repository from a GitHub "template repository".

Notes

At the time of writing (2020-10), v3 API support for template repo features requires that the following HTTP header be provided (as the developer's acknowledgement that the features are experimental):

    Accept: application/vnd.github.baptiste-preview+json

The user would be specifying the template repository to use in the TMPL_REPO_NAME argument. As noted in the GH v3 API docs, that value can be sanity checked to confirm that it really does represent a "template repository" by checking the repo's is_template property.

Some notes based on some empirical testing in the GitHub web UI:

• Not all repository attributes are copied verbatim into the newly created repo
• Attributes that are copied include:
  • Repo description
  • Default branch name
  • Files and directory structure
• Attributes that are not copied include:
  • Public/Private visibility setting (though there is a private parameter to explicitly set this in the API)
  • Issues enabled/disabled flag
  • Projects enabled/disabled flag
  • Wiki enabled/disabled flag
  • Issue "Labels" (Customized labels in the template repository are not present in the newly created repo)
• Ambiguous/Requires further testing
  • Branches other than the default. (The web UI has a checkbox to include them, but it is not checked by default. There does not seem to be a corresponding parameter in the v3 API for this decisions; need to find out what the behavior is.)