pullrequests: introduce limit to stop displaying additional changes

We've noticed some scalability issues when many descendants exist for the
changesets in a pull request.

To resolve this issue, we instead do not display any additional changes at all
if the amount of additional changes is larger than a configured limit.

(This occurred because we were merging a lot of heads in the repository.)

Throwing away some of the changesets would also keep the total amount more
manageable, but this would result in an (for the user) unpredictable subset of
changesets being shown.

This could be extended further with "too long to be shown - click here to
show", but that was quite a bit of additional work and did not cover our use
case of not allowing the display at all in case of too many additional changes.