nametrans rules can lead to different visiblename names for the
top-level directory, specifically both '.' and '' (the latter was
recently introduced). However, we need to be able to compare folder
names to see if we need to create a new directory or whether a directory
already exists, so we need to be able to compare a repositories
visiblename (=transposed via nametrans rule) with another folder.
To make the top-level directory comparison happen, we enforce a
top-level name of '', so that comparisons work.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>