Release notes are wrong for backported releases

[Krastanov]

Consider the following story for some package:

• release 1.0 is made
• the master branch is dedicated now to release 2.0 and PRs a,b&c happen.
• only PR c gets backported to a separate branch which is used to release 1.0.1

The autogenerated release notes for 1.0.1 should include only PR c, but instead they include PRs a,b&c

Seen here https://github.com/JuliaCollections/DataStructures.jl/issues/865

[lgoettgens]

Duplicate of https://github.com/JuliaRegistries/TagBot/issues/181