Allow for merge-base commit comparison on PR tagged events

[filipecosta90]

Right now we have 3 ways of triggering benchmarks:
-1 Pushes to repo
-2 Labeled PRs on repo ( fetchs fork code )
-3 historical data via cli

On option 2 we want to be able to compare vs merge-base commit.
Portion of event:

{
    "ref": "refs/heads/unstable.55555",
    "before": "572abee6f17753fe2af59aeb7c3dd42baee66e1b",
    "after": "921489d5392a13e10493c6578a27b4bd5324a929",
(...)

and to potentially also to a list of branch/version tags.