The pollution comes IMHO not only from having the timestamps in the commit (and to fix that the idea of using git-notes that came up in a sibling comment seems

The pollution comes IMHO not only from having the timestamps in the commit (and to fix that the idea of using git-notes that came up in a sibling comment seems to be a perfect fit), but more from trying to put too much in a single commit. This could easily be fixed by using a feature branch, do nice self-contained and easy to read commits in the branch, then attach the time-logging and billing information to the merge commit (using git-notes).