diff: fix --exit-code with external diff

diff: fix --exit-code with external diff

You can ask the diff machinery to let the exit code indicate whether
there are changes, e.g. with --exit-code.  It as two ways to calculate
that bit: The quick one assumes blobs with different hashes have
different content, and the more elaborate way actually compares the
contents, possibly applying transformations like ignoring whitespace.

Always use the slower path by setting the flag diff_from_contents,
because any of the files could have an external diff driver set via an
attribute, which might consider binary differences irrelevant, like e.g.

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