Make sure that all the changes since the last release are associated to the milestone

[morozov]

Right now, for a given pull request, the target branch and the associated milestone are totally independent. It allows for a situation when some changes are released as part of the changes in the branch being tagged but are not listed in the release note. It happens every once in a while.

High-level, the logic is the following:

1. Take all merge commits since the last release.
2. Associate them with pull requests on GitHub and the corresponding issues (via the Fixes and Closes references).
3. If there's a discrepancy, report a problem and halt.
4. Additionally, if some changes need to be excluded from the notes explicitly, there may be a label for that.

[morozov]

For the reference: doctrine/dbal#3790 (comment) (happened in a manually managed release).

[Ocramius]

Moved to laminas/automatic-releases#27