Merge changelog entries for fixups with main PR

[straight-shoota]

The release changelog directly maps all merged PRs to changelog entries. Some PRs are bugfixes or enhancements to other changes. They're less relevant for users because they only affect an intermediary state in the release process. For example, #15178 fixes a regression, but that regression is only in master.
The changelog describes the difference between two releases. So a reader wouldn't even consider this a regression fix because the code that introduced the bug was never released on its own without the fix.

From the perspective of the changelog it makes sense to group related PRs that build on top of each other into a single changelog entry. We had been doing this already previously, when we were writing the changelog manually.

This patch adds support for this to the changelog generator. If a PR title contains the text [fixup #XXXX] (where #XXXX references a PR), its entry gets merged with the referenced PR. The title and labels of the fixup are ignored, only the PR number and author show up in the changelog.

Example diff that this change would produce in the changelog for 1.15:

-- *(interpreter)* setup signal handlers in interpreted code ([#14766], thanks @ysbaddaden)
-- Add compiler versions constraints for interpreter signal handler [fixup #14766] ([#15178], @straight-shoota)
+- *(interpreter)* setup signal handlers in interpreted code ([#14766], [#15178], thanks @ysbaddaden, @straight-shoota)

[Sija]

Could be simplified by using Set.

Suggested change

	authors = [] of String
	pull_requests.each do |pr|
	author = pr.author || next
	authors << author unless authors.includes?(author)
	end
	authors = Set(String).new
	pull_requests.each do |pr|
	authors << pr.author || next
	end