util: checknlink should remove file it creates if an exception occurs There's no reason to leave the file behind.

merge: remove files with extra actions from merge action list See the comment for a detailed explanation why. Even though this is a bug, I've sent it to 'default' rather than 'stable' because it isn't triggered in any code paths in stock Mercurial, just with the merge driver included. For the same reason I haven't included any tests here -- the merge driver is getting a new test.