gitweb: fix commitdiff for root commits

After changing all "-|" open invocations to list form, commitdiff for
initial commit (without parent) got broken; it returned incorrectly
empty patch earlier.  Use '--root' option to git-diff-tree for initial
(root) commit.

No checking for empty $hash_parent in git_commitdiff_plain -- we rely
on gitweb to give correct parameters for commitdiff_plain action.

Noticed by Matthias Lederhofer (matled).

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

gitweb/gitweb.perl

@@ -2109,7 +2109,7 @@ sub git_commitdiff {
 		die_error(undef, "Unknown commit object");
 	}
 	if (!defined $hash_parent) {
-		$hash_parent = $co{'parent'};
+		$hash_parent = $co{'parent'} || '--root';
 	}
 	open my $fd, "-|", $GIT, "diff-tree", '-r', $hash_parent, $hash
 		or die_error(undef, "Open git-diff-tree failed");