diff --quiet

diff --quiet

This adds the command line option 'quiet' to tell 'git diff-*'
that we are not interested in the actual diff contents but only
want to know if there is any change.  This option automatically
turns --exit-code on, and turns off output formatting, as it
does not make much sense to show the first hit we happened to
have found.

The --quiet option is silently turned off (but --exit-code is
still in effect, so is silent output) if postprocessing filters
such as pickaxe and diff-filter are used.  For all practical
purposes I do not think of a reason to want to use these filters
and not viewing the diff output.

The backends have not been taught about the option with this patch.
That is a topic for later rounds.

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