Sprint Report calculation incorrectly includes cloned issues

Reproduce:

• Create a sprint and assign some issues to it.
• Clone one of the issues and move the cloned issue back to the backlog
• Start and complete the sprint
• The sprint report mentions the cloned issue as not completed in the issue table

Problem is in BurndownUtils#addSprintHistoryChanges method. The code makes the assumption that the first change the sprint custom field value of an issue is setting the value to a sprint, not removing it, as is the case with the cloned issue. This causes the the cloned issue to be counted as being part of the sprint, while it was not.