Don't double-slash filenames at the root.
Replace either the end of the dir string that isn't preceded by a slash or the slash at the end of the dir string with a slash and the filename.
This prevents the root directory from ending up as two slashes, which breaks files_versions on Windows servers. See #4079 for an explanation.
Merge pull request #5214 from owncloud/no_extra_slashes

Don't double-slash filenames at the root.