Get "Merge when build succeeds" to work when commits were pushed to MR
target branch while builds were running

The Merge when build succeeds service only merges when the MR is
mergeable (open, not WIP, no conflicts).

When the target branch is updated, all affected MRs have their merge
status set to `unchecked`, and the conflicts check will only happen
when `check_if_can_be_merged` is called, which happens when the MR page
is viewed.

When someone enables the automatic merge, the target branch is updated,
no-one views the MR page again, and the build succeeds, the mergeability
check will fail and the MR will not in fact be merged.

This MR makes sure `check_if_can_be_merged` is always called when MR
mergeability is checked.

See merge request !2304