diff --cc: a lost line at the beginning of the file is shown incorrectly

diff --cc: a lost line at the beginning of the file is shown incorrectly

When combine-diff inspected the diff from one parent to the merge result,
it misinterpreted a header in the form @@ -l,k +0,0 @@.

This hunk header means that K lines were removed from the beginning of the
file, so the lost lines must be queued to the sline that represents the
first line of the merge result, but we incremented our pointer incorrectly
and ended up queuing it to the second line, which in turn made the lossage
appear _after_ the first line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>