commit-reach(repo_get_merge_bases_many_dirty): pass on errors

commit-reach(repo_get_merge_bases_many_dirty): pass on errors

(Actually, this commit is only about passing on "missing commits"
errors, but adding that to the commit's title would have made it too
long.)

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

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>