[tz] Breach of tzdb charter: Merging timezones is not within the charter

[Paul Eggert]

On 6/2/21 7:16 PM, Tom Lane via tz wrote:
> I just experimented and confirmed that including backzone isn't nearly
> sufficient to bring the output data back to what it was previously.

It is sufficient, in the sense that if you included backzone before and 
still include it now, there are no changes to any timestamps (either 
pre- or post-1970). I've tested this.

If you want only some of backzone but not all of it, we could add 
support for that (as I mentioned earlier). With that support, you could 
say "I want backzone to supply Europe/Stockholm", or "I want the zones 
that changed in 2021b to come from backzone". This should handle the use 
case you described.