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

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

Closes #2029.

See merge request !1558

app/services/merge_requests/build_service.rb

@@ -16,9 +16,6 @@ module MergeRequests
         return build_failed(merge_request, nil)
       end
 
-      # Generate suggested MR title based on source branch name
-      merge_request.title = merge_request.source_branch.titleize.humanize
-
       compare_result = CompareService.new.execute(
         current_user,
         merge_request.source_project,
@@ -52,6 +49,14 @@ module MergeRequests
         merge_request.compare_failed = false
       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
+
       merge_request
 
     rescue Gitlab::Satellite::BranchesWithoutParent