Merge branch 'rs-merge_commit_message' into 'master'

Improve how MergeCommit#merge_commit_message builds the message

Now a merge request with a blank description will no longer produce a
merge commit message like this:

```
Merge branch 'foo' into 'master'

Bring the wonders of foo into the world



See merge request !7283
```

What an improvement! 

See merge request !6450