vfs: add a 'reljoin' function for joining relative paths The vfs.join method only works for absolute paths. We need something that works for relative paths too when transforming filenames. Since os.path.join may misbehave in tricky encoding situations, encapsulate the new join method in our vfs abstraction. The default implementation remains os.path.join, but this opens the door to other VFSes doing something more intelligent based on their needs. In the same go, we replace the usage of 'os.path.join' in transaction code.