add--interactive: detect bogus diffFilter output

add--interactive: detect bogus diffFilter output

It's important that the diff-filter only filter the
individual lines, and that there remain a one-to-one mapping
between the input and output lines. Otherwise, things like
hunk-splitting will behave quite unexpectedly (e.g., you
think you are splitting at one point, but it has a different
effect in the text patch we apply).

We can't detect all problematic cases, but we can at least
catch the obvious case where we don't even have the correct
number of lines.

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