unpack_loose_rest(): simplify error handling

unpack_loose_rest(): simplify error handling

Inflating a loose object is considered successful only if we got
Z_STREAM_END and there were no more bytes. We check both of those
conditions and return success, but then have to check them a second time
to decide which error message to produce.

I.e., we do something like this:

  if (!error_1 && !error_2)
          ...return success...

  if (error_1)
          ...handle error1...
  else if (error_2)
          ...handle error2...
  ...common error handling...

This repetition was the source of a small bug fixed in an earlier commit
(our Z_STREAM_END check was not the same in the two conditionals).

Instead we can chain them all into a single if/else cascade, which
avoids repeating ourselves:

  if (error_1)
          ...handle error1...
  else if (error_2)
          ...handle error2....
  else
          ...return success...
  ...common error handling...

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>