hash-object: -- and --help

hash-object: -- and --help

It was cumbersome to feed hash-object the file '-t' (you could
have said "./-t", though).  Teach it '--' that terminates the
option list, like everybody else.  There is no way to extract
usage string from the command either, so teach it "--help" as
well.

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