While git respects a user's local timezone and returns
seconds-since-the-Epoch, we were unnecessarily and incorrectly
calling gmtime+strftime on its result. So ignore calling
gmtime+strftime when the strftime format is "%s", just feed
the output time from git directly to Xapian.
This is mainly for lei, which will likely run in a variety of
timezones. While we're at it, add a recommendation to use
TZ=UTC in public-inbox-httpd, in case there are (misguided :P)
sysadmins who set a non-UTC TZ.