Re: "git shortlog -sn --follow -- <path>" counts all commits to entire repo

Jeff King [off-list ref] writes:

Yeah. It depends on exactly how such a fix is made. I think one
improvement would be to actually bump --follow handling into the
limit_list() stage, so that we properly handle history simplification
over followed paths. In which case get_revision() would just never
return the uninteresting commits, and the current shortlog code would
Just Work.

That said, I don't think we can go wrong by making shortlog's traversal
more like log's. Any changes we make to --follow will be aimed at and
tested with git-log, so the more code they share the more likely it is
that shortlog won't bitrot.

Both true.  

Using log-tree traversal machinery instead of just get_revision()
would probably mean we would slow it down quite a bit unless we are
careful, but at the same time, things like "git shortlog -G<string>"
would suddenly start working, so this is not just helping the
"--follow" hack.