Support `only: changes:` on MR pipelines

The current implementation of only: changes: does not work for new branches. It is due to inability to understand all changes.

The introduction of Merge Request Pipelines allows us to get a proper changeset of changes: before_sha and sha of changes:

1. before_sha to be merge-base of changes between our source and target branch,
2. sha head of the branch.

By looking at before_sha and sha we should fix support for running jobs only when given files are changed, thus solving the biggest deficiency of current implementation.

What needs to be done? I think that we need to set proper before_sha when we create a new MR pipelines (CreatePipelineService). This should self-resolve the only: changes: behavior.