merge-base --octopus: reduce the result from get_octopus_merge_bases()

merge-base --octopus: reduce the result from get_octopus_merge_bases()

Scripts that use "merge-base --octopus" could do the reducing
themselves, but most of them are expected to want to get the reduced
results without having to do any work themselves.

Tests are taken from a message by Василий Макаров
<einmalfel@gmail.com>

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

 We might want to vet the existing callers of the underlying
 get_octopus_merge_bases() and find out if _all_ of them are doing
 anything extra (like deduping) because the machinery can return
 duplicate results. And if that is the case, then we may want to
 move the dedupling down the callchain instead of having it here.