Resolve "Duplicate pipelines"

Closes #214 (closed)

This merge request fixes the issue of duplicate pipelines being created for a commit if there's a merge request open for it.

This also means that the Run pipeline button on Pipelines page will not work for branches with associated merge requests. In order to manually rerun a pipeline for such a branch, please use the Run pipeline on the Pipelines tab on the associated merge request's details page (eg. !153).

This merge request also fixes the issue with the second pipeline for the same commit always failing.