date: check date overflow against time_t

When we check whether a timestamp has overflowed, we check
only against ULONG_MAX, meaning that strtoul has overflowed.
However, we also feed these timestamps to system functions
like gmtime, which expect a time_t. On many systems, time_t
is actually smaller than "unsigned long" (e.g., because it
is signed), and we would overflow when using these
functions.  We don't know the actual size or signedness of
time_t, but we can easily check for truncation with a simple
assignment.

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

date.c

@@ -1085,3 +1085,20 @@ unsigned long approxidate_careful(const char *date, int *error_ret)
 	gettimeofday(&tv, NULL);
 	return approxidate_str(date, &tv, error_ret);
 }
+
+int date_overflows(unsigned long t)
+{
+	time_t sys;
+
+	/* If we overflowed our unsigned long, that's bad... */
+	if (t == ULONG_MAX)
+		return 1;
+
+	/*
+	 * ...but we also are going to feed the result to system
+	 * functions that expect time_t, which is often "signed long".
+	 * Make sure that we fit into time_t, as well.
+	 */
+	sys = t;
+	return t != sys || (t < 1) != (sys < 1);
+}