diff: handle diffstat of rewritten binary files

diff: handle diffstat of rewritten binary files

The logic in builtin_diffstat assumes that a
complete_rewrite pair should have its lines counted. This is
nonsensical for binary files and leads to confusing things
like:

  $ git diff --stat --summary HEAD^ HEAD
   foo.rand |  Bin 4096 -> 4096 bytes
   1 files changed, 0 insertions(+), 0 deletions(-)

  $ git diff --stat --summary -B HEAD^ HEAD
   foo.rand |   34 +++++++++++++++-------------------
   1 files changed, 15 insertions(+), 19 deletions(-)
   rewrite foo.rand (100%)

So let's reorder the function to handle binary files first
(which from diffstat's perspective look like complete
rewrites anyway), then rewrites, then actual diffstats.

There are two bonus prizes to this reorder:

  1. It gets rid of a now-superfluous goto.

  2. The binary case is at the top, which means we can
     further optimize it in the next patch.

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