Enable code checks or linters when files from tools are changed (i.e. run all PHPUnit checks when phpunit.xml.dist is the only file within the diff)

[boesing]

Feature Request

Q	A
New Feature	yes

Summary

Currently, if the diff only contains a change for phpunit.xml.dist or infection.json (etc.), no checks would be generated.
With #83 it would be quite easy to enable specific tool checks for the diff.

Maybe this issue should be renamed to "only enable checks for tools which do match the diff (if a diff is available)". For now, this Ticket is just to keep track of ideas which came up during the development of #83.