diff-highlight: avoid highlighting combined diffs

diff-highlight: avoid highlighting combined diffs

The algorithm in diff-highlight only understands how to look
at two sides of a diff; it cannot correctly handle combined
diffs with multiple preimages. Often highlighting does not
trigger at all for these diffs because the line counts do
not match up.  E.g., if we see:

  - ours
   -theirs
  ++resolved

we would not bother highlighting; it naively looks like a
single line went away, and then a separate hunk added
another single line.

But of course there are exceptions. E.g., if the other side
deleted the line, we might see:

  - ours
  ++resolved

which looks like we dropped " ours" and added "+resolved".
This is only a small highlighting glitch (we highlight the
space and the "+" along with the content), but it's also the
tip of the iceberg. Even if we learned to find the true
content here (by noticing we are in a 3-way combined diff
and marking _two_ characters from the front of the line as
uninteresting), there are other more complicated cases where
we really do need to handle a 3-way hunk.

Let's just punt for now; we can recognize combined diffs by
the presence of extra "@" symbols in the hunk header, and
treat them as non-diff content.

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