diff: skip implicit no-index check when given --no-index

diff: skip implicit no-index check when given --no-index

We can invoke no-index mode in two ways: by an explicit
request from the user, or implicitly by noticing that we
have two paths, and at least one is outside the repository.

If the user already told us --no-index, there is no need for
us to do the implicit test at all.  However, we currently
do, and downgrade our "explicit" to DIFF_NO_INDEX_IMPLICIT.

This doesn't have any user-visible behavior, though it's not
immediately obvious why. We only trigger the implicit check
when we have exactly two non-option arguments. And the only
code that cares about implicit versus explicit is an error
message that we show when we _don't_ have two non-option
arguments.

However, it's worth fixing anyway. Besides being slightly
more efficient, it makes the code easier to follow, which
will help when we modify it in future patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>