Block pull request merge if specific file line was changed.

[Jacek Gębal]

I'd like to block PR merges when a specific tag in one of two specific pom files was changed.

How would I block a PR merge if the line containing specific tag/text was modified?

Example: 

Files: /module1/pom.xml, /module2/pom.xml contain a preference:

<my_important_preference>VALUE</my_important_preference>

I would like to block a merge of PR if the line containing that preference was changed in any of those files.