merge: split up checks for unknown and ignored files that differ In some real-world cases it is preferable to allow overwriting ignored files while continuing to abort on unknown files. This primarily happens when we're replacing build artifacts (which are ignored) with checked in files, but continuing to abort on differing files that aren't ignored. We're redefining merge.checkunknown to only control the behavior for files that aren't ignored. That's fine because this config was only very recently introduced and has not made its way into any Mercurial releases yet.