repair: speed up stripping of many roots repair.strip() expects a set of root revisions to strip. It then builds the full set of descedants by walking the descandants of each. It is rare that more than a few roots get passed in, but if that happens, it will wastefully walk the changelog for each root. So let's just walk it once. I noticed this because the narrowhg extension was passing not only roots, but all the commits to strip. When there were tens of thousands of commits to strip, this resulted in quadratic behavior with that extension.