Change xdl_merge to generate output even for null merges

Change xdl_merge to generate output even for null merges

xdl_merge used to have a check to ensure that there was at least
some change in one or other side being merged but this suppressed
output for the degenerate case when base, local and remote
contents were all identical.

Removing this check enables correct output in the degenerate case
and xdl_free_script handles freeing NULL scripts so there is no
need to have the check for these calls.

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>