[tz] Java & Rearguard

[Steve Summit]

I wrote:
> Java/CLDR's mapping wants to be dst=0 -> GMT, dst=1 -> IST, which
> is of course incompatible with tzdb's current mapping.  But if
> Java/CLDR were to somehow change to dst=0 -> IST, dst=1 -> GMT,
> to match tzdb's current mapping, it would then start getting
> the wrong answer before 1970, because for dates before 1970
> (in 2019a, at least), tzdb uses dst=0 -> GMT, dst=1 -> IST.

Whoops, wait.  The pre-1970 "IST" is "Irish Summer Time", isn't it?
Never mind.  (Perils of relying on those pesky, ambiguous tz abbreviations.)