merge: while checking for unknown files don't follow symlinks (issue5027) Previously, we were using Python's native 'os.path.isfile' method which follows symlinks. In this case, since we're operating on repo contents, we don't want to follow symlinks. There's a behaviour change here, as shown by the second part of the added test. Consider a symlink 'f' pointing to a file containing 'abc'. If we try and replace it with a file with contents 'abc', previously we would have let it though. Now we don't. Although this breaks naive inspection with tools like 'cat' and 'diff', on balance I believe this is the right change.