combine-diff: lift 32-way limit of combined diff

combine-diff: lift 32-way limit of combined diff

The "raw" format of combine-diff output is supposed to have as many
colons as there are parents at the beginning, then blob modes for
these parents, and then object names for these parents.

We weren't however prepared to handle a more than 32-way merge and
did not show the correct number of colons in such a case.

Signed-off-by: Junio C Hamano <gitster@pobox.com>