blame: don't overflow time buffer

blame: don't overflow time buffer

When showing the raw timestamp, we format the numeric
seconds-since-epoch into a buffer, followed by the timezone
string. This string has come straight from the commit
object. A well-formed object should have a timezone string
of only a few bytes, but we could be operating on data
pushed by a malicious user.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>