Some improvements to Special:MergeHistory

* Added a 5000 revision limit.
* Also fixed the radio-button selection to reflect the URL
  params so the "unmerge" log link works as expected.
* Added some TODO comments.

bug: 66155
Change-Id: Ib70e9d58ae2de723b965e52c679c4e786016420e
Merge "Some improvements to Special:MergeHistory"