So don't commit irrelevant changes to your feature branch, this falls upon the "blind commits" -category of problem.
The best way to explain why certain changes were made is not in the commit messages as you should not review separate commits but the end result of the work in the feature branch.
What I do if some change is unclear is usually one of the following:
Oh and if you don't know which of the changes you made are relevant to your feature branch anymore you don't commit often enough.
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