xread: poll on non blocking fds

xread: poll on non blocking fds

The man page of read(2) says:

  EAGAIN The file descriptor fd refers to a file other than a socket
	 and has been marked nonblocking (O_NONBLOCK), and the read
	 would block.

  EAGAIN or EWOULDBLOCK
	 The file descriptor fd refers to a socket and has been marked
	 nonblocking (O_NONBLOCK), and the read would block.  POSIX.1-2001
	 allows either error to be returned for this case, and does not
	 require these constants to have the same value, so a portable
	 application should check for both possibilities.

If we get an EAGAIN or EWOULDBLOCK the fd must have set O_NONBLOCK.
As the intent of xread is to read as much as possible either until the
fd is EOF or an actual error occurs, we can ease the feeder of the fd
by not spinning the whole time, but rather wait for it politely by not
busy waiting.

We should not care if the call to poll failed, as we're in an infinite
loop and can only get out with the correct read().

Signed-off-by: Stefan Beller <sbeller@google.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>