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

The `paint_down_to_common()` function was just taught to indicate
parsing errors, and now the `merge_bases_many()` function is aware of
that, too.

One tricky aspect is that `merge_bases_many()` parses commits of its
own, but wants to gracefully handle the scenario where NULL is passed as
a merge head, returning the empty list of merge bases. The way this was
handled involved calling `repo_parse_commit(NULL)` and relying on it to
return an error. This has to be done differently now so that we can
handle missing commits correctly by producing a fatal error.

Next step: adjust the caller of `merge_bases_many()`:
`get_merge_bases_many_0()`.

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

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