When the source file is empty, the calculation of the merge score
results in a division by zero. In the situation:
== preimage == == postimage ==
F (empty file) F (a large file)
E (a new empty file)
it does not make sense to consider F->E as a rename, so it is better not
to break the pre- and post-image of F.
Bail out early in this case to avoid hitting the divide-by-zero. This
causes the merge score to be left at zero.
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>