fast-import: switch crash-report date to iso8601

fast-import: switch crash-report date to iso8601

When fast-import emits a crash report, it does so in the
user's local timezone. But because we omit the timezone
completely for DATE_LOCAL, a reader of the report does not
immediately know which time zone was used. Let's switch this
to ISO8601 instead, which includes the time zone.

This does mean we will show the time in UTC, but that's not
a big deal. A crash report like this will either be looked
at immediately (in which case nobody even looks at the
timestamp), or it will be passed along to a developer to
debug, in which case the original timezone is less likely to
be of interest.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>