Improve the regular expression verifications for push rules

Reported here: #463542 (comment 1930450440)

Problem

While setting up push rules regular expressions (for example Require expression in commit messages), it's not clear if they will work as intended.

Users have to manually test them by pushing commits to the repository until they get an expected result. This is a tedious process.

Proposal

It would be nice to have a UI tool to verify which commits messages match the regular expression.