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>

cache.h

@@ -909,6 +909,7 @@ void datestamp(char *buf, int bufsize);
 unsigned long approxidate_careful(const char *, int *);
 unsigned long approxidate_relative(const char *date, const struct timeval *now);
 enum date_mode parse_date_format(const char *format);
+int date_overflows(unsigned long date);
 
 #define IDENT_STRICT	       1
 #define IDENT_NO_DATE	       2