service: source pre_merge from the git repo

For Marge-like workflows, post-merge runs start their lives as pre-merge and become considered as post-merge only once the pipeline completed and gets merged in the default branch.

Let's drop the need for labelling branches/refs as pre/post-merge and instead check if the commit associated to the pipeline is now found in the default branch by checking if the merge base of the two refs is equal to the pipeline's commit.