How to get list of commits from Pull request in dry-run phase

[Kaushik Veluru]

I am writing a Merge Check hook to go through list of commits in a PR and parse the JIRA tickets in the commit messages and verify some data in JIRA. In dry-run phase, I am not able pullRequestMergeHookRequest.getRefChanges() always returns 0. So, how do I achieve this? 

I am using PullRequestMergeHookRequest because I want' this to be triggered when Merge button is clicked.