local_tzoffset: detect errors from tm_to_time_t

When we want to know the local timezone offset at a given
timestamp, we compute it by asking for localtime() at the
given time, and comparing the offset to GMT at that time.
However, there's some juggling between time_t and "struct
tm" which happens, which involves calling our own
tm_to_time_t().

If that function returns an error (e.g., because it only
handles dates up to the year 2099), it returns "-1", which
we treat as a time_t, and is clearly bogus, leading to
bizarre timestamps (that seem to always adjust the time back
to (time_t)(uint32_t)-1, in the year 2106).

It's not a good idea for local_tzoffset() to simply die
here; it would make it hard to run "git log" on a repository
with funny timestamps. Instead, let's just treat such cases
as "zero offset".

Reported-by: Norbert Kiesel <nkiesel@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -74,6 +74,8 @@ static int local_tzoffset(unsigned long time)
 	localtime_r(&t, &tm);
 	t_local = tm_to_time_t(&tm);
 
+	if (t_local == -1)
+		return 0; /* error; just use +0000 */
 	if (t_local < t) {
 		eastwest = -1;
 		offset = t - t_local;

t/t0006-date.sh

@@ -48,6 +48,11 @@ check_show default "$TIME" 'Wed Jun 15 16:13:20 2016 +0200'
 check_show raw "$TIME" '1466000000 +0200'
 check_show iso-local "$TIME" '2016-06-15 14:13:20 +0000'
 
+# arbitrary time absurdly far in the future
+FUTURE="5758122296 -0400"
+check_show iso       "$FUTURE" "2152-06-19 18:24:56 -0400"
+check_show iso-local "$FUTURE" "2152-06-19 22:24:56 +0000"
+
 check_parse() {
 	echo "$1 -> $2" >expect
 	test_expect_${4:-success} "parse date ($1${3:+ TZ=$3})" "