Having this in a concern allows us to reuse it for different single
purpose classes that call out to git without going through the
repository every time.
The previous implementation iterated across the entire patch set
to determine the number of lines added, deleted, and changed. Rugged
has a native method `Rugged::Diff#stat` that does this already,
which appears to be a little faster and require less RAM than doing
this ourselves.
Improves performance in #41524