When specifying a since-tag that corresponds to a PR merge commit (where the ref of the tag and the merge commit are equal) the PR is unexpectedly included in the changelog.

Tested on latest pre-release build using

github_changelog_generator -u ppy -p osu-framework --since-tag changelog-20180120 --max-issues 500 --no-issues --release-branch master

note that the state of this repository may change, let me know if i need to make an isolated test case.