regression in _FileInFile which is used in file-like objects returned
by TarFile.extractfile(). The inefficient design of the
_FileInFile.read() method causes various dramatic side-effects and
errors:

  - The data segment of a file member is read completely into memory
    every(!) time a small block is accessed. This is not only slow
    but may cause unexpected MemoryErrors with very large files.
  - Reading members from compressed tar archives is even slower
    because of the excessive backwards seeking which is done when the
    same data segment is read over and over again.
  - As a backwards seek on a TarFile opened in stream mode is not
    possible, using extractfile() fails with a StreamError.