Feature summary (what you would like to be able to do and where):
The Special:MergeHistory interface should allow merging all edits from time T1 to time T2, rather than only from the beginning of time to time T2.
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
This is kind of a poor man's version of T23312 (but hopefully much easier to implement, and something I may tackle myself whereas building a a new system entirely is not). It would also make it possible to separate recent edits from huge pages without moving the entire early history of the page, and probably a bunch of other things I haven't thought of.
Benefits (why should this be implemented?):
See above. May also help with situations like https://en.wikipedia.org/wiki/Wikipedia_talk:Administrators%27_noticeboard/Archive_17#Significant_100+_edits_wiped_out_from_WP:AN (although my views on using XML import to rebase edits instead of revdelling seem to be against consensus right now)