Previously join_paths() was incorrectly handling absolute paths, for
example join_paths('a/b', '/c/d') would return 'a/b/c/d' rather than
'/c/d'. This was a problem when following a symlink in
find_stowed_path(), because if the symlink was not owned by Stow and
pointed to an absolute path, find_stowed_path() might accidentally
deem the link owned by Stow, if c/d was a valid path relative to the
current directory.
