filemerge: add a hacktastic version of internal:merge3 for merge diffs This is a version of merge3 that always reports success, so that conflict markers get preserved without us having to implement conflict storage for in-memory merge. Credit to martinvonz for the idea. The only planned consumer of this "merge tool" is my upcoming merge-diffs functionality, though I suspect it could be useful in other ways. Differential Revision: https://phab.mercurial-scm.org/D8515