diff: respect textconv in rewrite diffs

diff: respect textconv in rewrite diffs

Currently we just skip rewrite diffs for binary files; this
patch makes an exception for files which will be textconv'd,
and actually performs the textconv before generating the
diff.

Conceptually, rewrite diffs should be in the exact same
format as the a non-rewrite diff, except that we refuse to
share any context. Thus it makes very little sense for "git
diff" to show a textconv'd diff, but for "git diff -B" to
show "Binary files differ".

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