Improve handling of hunk-header when merging.

Improve handling of hunk-header when merging.

If we are merging a patch, the patch will contain a number of
hunk-headers (essentially the @@ NN,MM @@) lines.

These should never appear in the merge, and must always line up with
end-of-line in the original as they represent a number of lines having
been deleted.

So:
  Cause these to each create a simple Extraneous merger, never
    combined with adjacent texts

  exclude them from any conflicts in isolate_conflict, so they
    can never appear in a merge

  Ensure that a preceeding 'Unmatched' finishes on an end-of-line.

Also update lots of self tests with improved results.

Signed-off-by: NeilBrown <neilb@suse.de>