diff-ni: fix the diff with standard input

diff-ni: fix the diff with standard input

The earlier commit to read from stdin was full of problems, and
this corrects them.

 - The mode bits should have been set to satisify S_ISREG(); we
   forgot to the S_IFREG bits and hardcoded 0644;
 - We did not give escape hatch to name a path whose name is
   really "-".  Allow users to say "./-" for that;
 - Use of xread() was not prepared to see short read (e.g. reading
   from tty) nor handing read errors.

Signed-off-by: Junio C Hamano <junkio@cox.net>