Look up release branch in commit comment

[christopher-dG]

TagBot/tagbot/action/repo.py

Lines 151 to 154 in bb54f2e

	@property
	def _release_branch(self) -> str:
	"""Get the name of the release branch."""
	return self.__release_branch or self._repo.default_branch

The code above means "Static release branch set in TagBot config if it's set, otherwise the repo's default branch". It's ignoring release branch info from a Registrator invocation like @JuliaRegistrator register branch=foo.

The actual effect of this is we miss out on the nice "N commits since this release" for those latter releases.

The priority for release branch selection should be:

1. Branch specified by Registrator invocation
2. Release branch specified in TagBot config
3. Default branch

Right now we're just missing (1). We can look up the commit comment by reading the registry PR body, but we're out of luck for the web UI.

I won't get to this soon but I'll keep it in mind for the rewrite ™️