[tz] Was there a 1971-12-31 23:59:60?

[Paul Eggert]

Paul Gilmartin via tz wrote:
> "leap-seconds.list" shows one more leap second that "leapseconds".

No, because the file "leap-seconds.list" is not merely a list of leap seconds; 
it is a list of corrections to apply to TAI to get modern UTC, which is a 
somewhat different thing. That file's first entry is the first such correction 
to apply, and this correction is not a leap second.

I installed the attached patch into the tzdb development version to try to 
clarify this in the tzdb "leapseconds" file, without going into too much detail.
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