Merge history (added in MW 1.12) is a special page that makes it possible to automatically merge the history of two pages.
Pages merged through this special pages is logged in a "merge log".
Functionality and use
To merge the history of two pages, go to Special:MergeHistory. In the source page field, put the page which you want the history of to be merged into another page. In the destination page field, put the page which you want the history of the source page to be merged into. Then, click "show mergeable edits," which will display a list of all edits to the source page which have the ability of being merged into the destination page. Only revisions made to the destination page before the source page was created can be merged, and thus will be the only ones shown. If "Merge revisions" is clicked when edits have been made to the destination page after the creation of the source page, an error message will appear; you must select from the available mergeable revisions, which in most cases, will be all of them.