Re: [PATCH v3 8/9] odb: return temporary ODB source when set

> When invoked, `odb_set_temporary_primary_source()` installs a temporary
> object directory as the new primary ODB source. A caller that wants to
> operate on the ODB source of the open transaction must assume that it is
> the first entry in the ODB source list which is a bit awkward and
> fragile.
> 
> Instead, return the newly installed source directly and report the
> previous primary source via a new `prev_source` out parameter. Propagate
> the installed source through `tmp_objdir_replace_primary_odb()` and
> start storing it in the "files" ODB transaction so a subsequent commit
> can easily access it without relying on the ODB source list ordering.