cherry-pick: handle root commits with external strategies

cherry-pick: handle root commits with external strategies

The merge-recursive strategy already handles root commits;
it cherry-picks the difference between the empty tree and
the root commit's tree.

However, for external strategies, we dereference NULL and
segfault while building the argument list. Instead, let's
handle this by passing the empty tree sha1 to the merge
script.

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