When we push to an MR's target branch, we check if the MR's HEAD commit is
contained in the push. This lets us mark MRs as merged if they were merged
manually.

However, we also added a feature where you can create an empty MR from an
issue. If that MR is created around the time of a merge to the default branch,
we would process the push after creating the MR, and consider it to be a manual
merge.

To fix that, we exclude empty MRs from this process. If they are empty, they
were empty before the push we're processing, so we shouldn't touch them!