UNIX reference time of 1970-01-01 00:00 is UTC timezone, not local time zone

UNIX reference time of 1970-01-01 00:00 is UTC timezone, not local time zone

I got bitten because in the UK (where one would expect 1970-01-01 00:00
to be UTC 0) some politicians decided to mess around with daylight
savings time from 1968 to 1971; it was permanently BST (+0100).  That
means that on my computer the following is true:

	$ date --date="1970-01-01 00:00" +"%F %T %z (%Z)"
	1970-01-01 00:00:00 +0100 (BST)

This of course means that the --date argument to date is specified in
local time, not UTC.  So when the hooks--update script does this:

	date=$(date --date="1970-01-01 00:00:00 $ts seconds")

It's actually saying (in my timezone) "1970-01-01 01:00:00 UTC" + $ts.
Clearly this is wrong.  The UNIX epoch started at midnight UTC not 1am
UTC.

This leads to the tagged time in hooks--update being shown as one hour
earlier than the true tagged time (in my timezone).  The problem would
be worse for other timezones.  For a +1300 timezone on 1970-01-01, the
tagged time would be 13 hours earlier.  Oops.

The solution is to force the reference time to UTC, which is what this
patch does.  In my timezone:

	$ date --date="1970-01-01 00:00 +0000" +"%F %T %z (%Z)"
	1970-01-01 01:00:00 +0100 (BST)

Much better.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>