commit-reach(get_octopus_merge_bases): pass on "missing commits" errors

commit-reach(get_octopus_merge_bases): pass on "missing commits" errors

The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases()` function (which is also
surfaced via the `get_merge_bases()` macro) is aware of that, too.

Naturally, the callers need to be adjusted now, too.

Next step: adjust `repo_get_merge_bases_many()`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>