Under Manage>Troubleshooting> User Activity Log
1) Select All users
2) Select IPS Policy from the list
3) max events is 50000
4) set time period
You will get a list of changes on policies/rule sets/interfaces.
For policy/rule set changes, you can click on the policy change (column on the right hyperlink) which will then bring up more details, including the commit comments (if any )