If I tar up a directory foo, the tarfile contents usually look like

    foo/
    foo/file1
    foo/subdir1/
    foo/subdir1/file1.1

tar-mode lets me rename directories, but if I move point to the first
line and enter 

  R C-a C-k bar RET

it only updates the path for the first element, i.e.,

    bar
    foo/file1
    foo/subdir1/
    foo/subdir1/file1.1

It would be more useful to update all the entries that were under foo,
i.e.,

    bar/
    bar/file1
    bar/subdir1/
    bar/subdir1/file1.1