diff --quiet: make it synonym to --exit-code >/dev/null

diff --quiet: make it synonym to --exit-code >/dev/null

The point of --quiet was to return the status as early as possible without
doing any extra processing.  Well behaved scripts, when they expect to run
many diff operations inside, are supposed to run "update-index --refresh"
upfront; we do not want them to pay the price of iterating over the index
and comparing the contents to fix the stat dirtiness, and we avoided most
of the processing in diffcore_std() when --quiet is in effect.

But scripts that adhere to the good practice won't have to pay any more
price than the necessary lstat(2) that will report stat cleanliness, as
long as only -q is given without any fancier diff options.

More importantly, users who do ask for "--quiet -M --filter=D" (in order
to notice only the deletion, not paths that disappeared only because they
have been renamed away) deserve to get the result they asked for, even it
means they have to pay the extra price; the alternative is to get a cheap
early return that gives a result they did not ask for, which is much
worse.

Signed-off-by: Junio C Hamano <gitster@pobox.com>