We're changing what's been done as part of ac939c458c, as we've
notcied issues using `github.event.pull_request.merge_commit_sha`.
Basically, whenever a force-push would happen, the reference of
merge_commit_sha wouldn't be updated, leading us to test PRs with the
old code. :-/
In order to get the rebase properly working, we need to ensure we pull
the hash of the commit as part of checkout action, and ensure
fetch-depth is set to 0.
Fixes: #7414
Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>
