diffoscope.diff: Improve FIFO writing robustness.

We used to give input to diff using FIFO objects.
fec9e97c changed this to avoid relying on
/dev/fd, which is a good change for portability.  Unfortunately, the
implementation has a few issues of its own:

* It feeds diff with normal files, rather than FIFOs.  This could cause
  trouble if diff tries to read farther than the underlying feeder
  processes have written.
* It uses threads to write to the pseudo-FIFO, but the file object is
  manipulated in multiple threads.  I suspect this is the primary cause of
  the segfaults observed in #852013.
* fd_from_feeder is decorated as a context manager, but it never yields
  anything, which is not how it's expected to be used.  I'm not sure this is
  causing any problems, but it makes it harder to reason about what's going
  on.

This commit introduces a new FIFOFeeder class.  It is wholly responsible for
creating and feeding the FIFO, so we don't have to pass file objects across
the thread boundary and risk segfaults.  It uses context management to tell
when the FIFO is no longer needed, so it can clean up nicely.

Signed-off-by: Chris Lamb <lamby@debian.org>
Signed-off-by: Mattia Rizzolo <mattia@debian.org>