path completion should favor displaying the actually completed part of the string rather than the prefix

Essentially, this is exactly the same issue as #9835, except that it affects path completion rather than object completion: when completing /some/very/very/long/path/with/subdirectories/{a,and,b}, ipython (7.0.1) currently truncates away the end of the paths, whereas it would be more useful to truncate away the common, beginning part.