git rev-parse --since=1970-01-01 does not work reliably

Hi,

git rev-parse --since=1970-01-01 (and other git commands that take
date string arguments like --since) may fail when --since=1970-01-01 is
given.  Whether it fails or not depends on current time and timezone data.
For example, "TZ=Europe/Paris git rev-parse --since=1970-01-01" fails two
hours a day (between 00:00 and 02:00 CET), and those who use more eastern
timezones are even less lucky.  In artificial timezones like UTC-24 it
always fails:

$ TZ=UTC-24 git rev-parse --since=1970-01-01
--max-age=18446744073709523490

The problem is that several internal git functions implicitly convert
time_t to unsigned long, so when time_t gets negative, all date string
processing breaks.


-- 
ldv