diff: Rename QUIET internal option to QUICK

diff: Rename QUIET internal option to QUICK

The option "QUIET" primarily meant "find if we have _any_ difference as
quick as possible and report", which means we often do not even have to
look at blobs if we know the trees are different by looking at the higher
level (e.g. "diff-tree A B").  As a side effect, because there is no point
showing one change that we happened to have found first, it also enables
NO_OUTPUT and EXIT_WITH_STATUS options, making the end result look quiet.

Rename the internal option to QUICK to reflect this better; it also makes
grepping the source tree much easier, as there are other kinds of QUIET
option everywhere.

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