gitweb: Fix error in git_patchset_body for deletion in merge commit

gitweb: Fix error in git_patchset_body for deletion in merge commit

Checking if $diffinfo->{'status'} is equal 'D' is no longer the way to
check if the file was deleted in result.  For merge commits
$diffinfo->{'status'} is reference to array of statuses for each
parent.  Use the fact that $diffinfo->{'to_id'} is all zeros as sign
that file was deleted in result.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>