merge: refactor unknown file conflict checking Previously, we would do a full working directory walk including unknown files to perform a merge. In many cases, this was painful because unknown files greatly outnumbered tracked files and generally had no useful effect on the merge. Here we instead wait until we find a file in the destination that's not tracked locally and detect if it exists and is not ignored. This is usually cheaper but can be -more- expensive in the case where we're adding a huge number of files. On the other hand, the cost of statting the new files should be dwarfed by the cost of eventually writing them. In this version, case collisions are detected implicitly by os.path.exists and wctx[f] lookup.