A limitation to 50 commits PRs was put in place to avoid rebasing
huge PRs (as a rebase means 1 merge + 1 commit *per source commit*),
however the way it was done would also limit regular merges, and the
way the limitation was implemented was not clear.
* explicitly check that limit in the rebase case
* make error message on PR sizes (rebase 50 or merge 250) clearer
* remove limit from commits-fetching (check it beforehand)
* add a test to merge >50 commits PRs
* fix the local implementation of pulls/:number/commits to properly
paginate
