When a commit could not be enriched, we want to return the original
commit.

When calling `Commit.lazy` we get a `BatchLoader` instance that
behaves like `nil`. So now we explicitly trigger the loading in batch
and replace the commit with the original commit if it could not be
loaded.

When a commit could not be enriched, we want to return the original
commit.

When calling `Commit.lazy` we get a `BatchLoader` instance that
behaves like `nil`. So now we explicitly trigger the loading in batch
and replace the commit with the original commit if it could not be
loaded.

This makes sure the merge request view can still be loaded for merge
requests for which the source branch became unavailable.