model: simplify how get_commits_stats task group on author

Avoid using the caching h.person . We want to get rid of the model
dependency on helpers.

The stats are persisted, and any temporary incorrectness in the long term
cached h.person will thus remain forever. It is thus arguably better to avoid
using it in this place.

get_commits_stats is also a long running task, so speed is not *that* critical.
And generally, processing commits in order will have a lot of the same
committers, so a local cache will have a good hit rate.

(Alternatively, h.person could perhaps be in user model ... but that's not how
it is now.)