previous behavior used `_valid_git_reference_check` which assumes the target repo is cloned and up-to-date in the current directory

Instead, always attempt to resolve refs with the GitHub API and only fallback on date parsing if it fails,
avoiding the need to clone the repo.

repo has been migrated from choldgraf/github-activity to executablebooks org

github-activity produces empty results when run on the old URL

follows conventions of markdown formatters such as prettier

ensures trailing newline in output as well, which some editors complain about when missing

Officially a style change, but some markdown parsers don't handle headings without surrounding space.
(this is unlikely to be an issue anymore,
but it was the case in some markdown processors in the Jupyter ecosystem for a while)

👌IMPROVE: add blank lines below headings

Remove no longer used code

useful for embedding changelog output as a subsection in existing documents

--heading-level=3 sets base heading level, i.e. increasing from (h1, h2) to (h3, h4)

tqdm.auto.tqdm produces widgets in notebooks and nice terminal progress bars in cli

additionally, send log messages to stderr so that rendered output can more easily be captured, e.g. github-activity | pbpaste