Alternative solutions
We could write a custom merge driver for RELEASE-NOTES which always puts ‘ours’ before ’theirs’,
but I’m not sure the result will justify the investment.
Probably overkill for MediaWiki but I'd like to mention the way that Python developers (CPython) manages their release notes, in case it might be useful.
The TL;DR is that each change has a unique valid reStructuredText file in a specific directory and then there is a tool to merge all changes when a release is made.
The full process from a contributor point of view is described in [1].
The tool used to both generate the change files and merge them into a release file is [2].
[2]
https://pypi.org/project/blurb/