Merge branch 'mr-body-single-commit' into 'master'

Base new MR description on commit description if there's only one.

Closes #2029, again.

See merge request !1561

app/services/merge_requests/build_service.rb

@@ -50,12 +50,13 @@ module MergeRequests
       end
 
       commits = merge_request.compare_commits
-      merge_request.title = \
-        if commits && commits.count == 1
-          commits.first.title
-        else
-          merge_request.source_branch.titleize.humanize
-        end
+      if commits && commits.count == 1
+        commit = commits.first
+        merge_request.title       = commit.title
+        merge_request.description = commit.description.try(:strip)
+      else
+        merge_request.title = merge_request.source_branch.titleize.humanize
+      end
 
       merge_request