Release notes script: add a way to "merge" multiple PRs into a single changelog entry

[fingolfin]

Sometimes a feature is implement via multiple PRs. It would be nice if there was a way to ensure a single changelog is added but referencing all the PRs.

E.g. PR oscar-system/Oscar.jl#4478 and oscar-system/Oscar.jl#4264 together implemented isomorphism(::Type{PermGroup}, ::WeylGroup). So they should be shown in a single changelog entry.

One way to do signal that could be to merge entries with identical description. So either identical title, or using a # Release notes section in the PR description.

[aaruni96]

I have been thinking about this, and I haven't been able to come up with a nice interface for this.

Current ideas:

• Do things manually: Add "release notes not needed" to all but one of the PRs, and for the last one, use "release notes use body", and manually mention the PRs in the release notes
• Keep track of identical release notes section, and prepend all the PRs to one of these (and drop the rest)
• Something else nicer?