I added also a check for m==0 because the original check went explicitly against a merge-tree which has a file in this directory/file conflict

BUT: during testing this I detected our isDirectoryFileConflict() was implemented wrong. Therefore I put the work on NameConflictTreeWalker into an own change. I am sure what is in that proposal is sub-optimal ... but it works. Let's focus on this change on DirCacheCheckout.