commit messages and tests survive the test of time therefore should have enough information for others to make sense. just having a good commit title isn't enough, include reference to the commit hashes which this commit builds on, not the immediate parents but the distant ones if any.
the rule of thumb is to imagine some new dev (or yourself) reading your code in future and by then you forgot everything about the code. the commit message and the tests should have enough information for the reader to quickly (in reasonable time) understand what's going on.