Clean up date and timezone handling.
We used to pass changelog dates around as a "unixtime timezone" string
containing a pair of encoded ints. Now, they get passed around as a
(unixtime, timezone) tuple of numbers, which makes much more sense.
1:3aa14bbc23d9
0:8633637036c1
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8633637036c1
8633637036c1+
8633637036c1
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merging file1
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