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! :tada:

1 files changed, 5 insertions, 7 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 2dcf7f89bfc..aec555dcec0 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -590,13 +590,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def merge_commit_message
-    message = "Merge branch '#{source_branch}' into '#{target_branch}'"
-    message << "\n\n"
-    message << title.to_s
-    message << "\n\n"
-    message << description.to_s
-    message << "\n\n"
-    message << "See merge request !#{iid}"
+    message = "Merge branch '#{source_branch}' into '#{target_branch}'\n\n"
+    message << "#{title}\n\n"
+    message << "#{description}\n\n" if description.present?
+    message << "See merge request #{to_reference}"
+
     message
   end