[tz] Undoing the effect of the new alike-since-1970 patch

[Stephen Colebourne]

On Sun, 6 Jun 2021 at 19:59, Paul Eggert <eggert at cs.ucla.edu> wrote:
> If that process is too much trouble for downstream users, we can arrange
> to generate the alternate source upstream. For example, we could ship a
> tarball with an altered tzdata.zi file (this is what Tom suggested).

As noted previously, there are two main groups in the debate - minimal
and maximal.

1) The minimal group believe TZDB should only focus on data after 1970
and use the minimum set of IDs necessary to achieve that (considering
the ID name to have no meaning or value pre-1970).

2) The maximal group believe TZDB should place greater value on
pre-1970 data and the name of the ID (considering that governmental
authorities are an important part of time-keeping).

It is possible to automatically derive the data for group 1 from the
data for group 2. The other way around is not possible.

I'm suggesting that the default data in the repo is for group 2 with a
way to derive group 1. This could result in two separate downloadable
tarballs.

I imagine the tasks would be:
- take the relevant parts of the `backzone` data and put it back in
the main files (Canada/Montreal would not be put back for example)
- add a new `idmerge` file identifying IDs that are the same post-1970
and which ID should be the one to have its history retained
- alter the makefile to have a "minimal" option to use the new `idmerge` file

An alternative approach that avoids the `idmerge` file would be for
the "minimal" option to simply strip all pre-1970 data

Stephen