User forked a repository on GitHub and wiki pages are also forked, user do changes to wiki, but there is no way to send pull request for wiki changes, but Wiki is just another sub repository in GitHub repo, so admin of original repository have to do merge manually.
source wiki: https://github.com/VadimPanasiuk/sevntu.checkstyle.wiki.git
target/original wiki: https://github.com/sevntu-checkstyle/sevntu.checkstyle.wiki.git
Source of wisdom for massive squash.
If wiki was forked from latest state of original wiki, you can do it a bit simpler: