* New class ApiMergeHistory handles action=mergehistory
* Merge History functionality moved from SpecialMergeHistory to
MergeHistory
* SpecialMergeHistory now uses MergeHistory for actual merging
* Unit tests and i18n messages for above
Bug: T69742
Change-Id: Ic5078307dae78a2b3687e34a5d0a584988d483a1