As per comment, seems like `--author` is not sufficient for
contributions to be counted.
Note that this also undoes the previous logic of setting this value
locally, as that would cause disruption to the local system if this
logic was ever run on a long-running system (such as, for instance, a
development machine)
Turns out they're not actually needed, because the implementation of
"finding all commits" just iterates over the lines of the `git log`
output, anyway - no need to actually traverse the tree.
I am _reasonably_ sure this means that it's no longer a pre-requisite
that the source repo has linear history, either. If I discover during
adding tests that that actually is the case, I'll re-add that note.
