Job triggered by pull request not compared against correct base

[eivindjahren]

Akin to #95, when a job is triggered by a pull request, the checkout action may merge. If the PR branch is behind, the resulting comparison is still done against the common base although the coverage report is generated after the merge.

I am using the following workflow file: https://github.com/equinor/ert/blob/main/.github/workflows/coverage.yml

Log of it running and causing incorrect comparison here: https://github.com/equinor/ert/runs/4459445221