diff: fix handling of binary rewrite diffs

diff: fix handling of binary rewrite diffs

The current emit_rewrite_diff code always writes a text patch without
checking whether the content is binary. This means that if you end up with
a rewrite diff for a binary file, you get lots of raw binary goo in your
patch.

Instead, if we have binary files, then let's just skip emit_rewrite_diff
altogether. We will already have shown the "dissimilarity index" line, so
it is really about the diff contents. If binary diffs are turned off, the
"Binary files a/file and b/file differ" message should be the same in
either case. If we do have binary patches turned on, there isn't much
point in making a less-efficient binary patch that does a total rewrite;
no human is going to read it, and since binary patches don't apply with
any fuzz anyway, the result of application should be the same.

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