fix approxidate parsing of relative months and years

These were broken by b5373e9. The problem is that the code
marks the month and year with "-1" for "we don't know it
yet", but the month and year code paths were not adjusted to
fill in the current time before doing their calculations
(whereas other units follow a different code path and are
fine).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

date.c

@@ -857,7 +857,9 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 	}
 
 	if (match_string(date, "months") >= 5) {
-		int n = tm->tm_mon - *num;
+		int n;
+		update_tm(tm, now, 0); /* fill in date fields if needed */
+		n = tm->tm_mon - *num;
 		*num = 0;
 		while (n < 0) {
 			n += 12;
@@ -868,6 +870,7 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 	}
 
 	if (match_string(date, "years") >= 4) {
+		update_tm(tm, now, 0); /* fill in date fields if needed */
 		tm->tm_year -= *num;
 		*num = 0;
 		return end;

t/t0006-date.sh

@@ -53,8 +53,8 @@ check_approxidate 10.minutes.ago '2009-08-30 19:10:00'
 check_approxidate yesterday '2009-08-29 19:20:00'
 check_approxidate 3.days.ago '2009-08-27 19:20:00'
 check_approxidate 3.weeks.ago '2009-08-09 19:20:00'
-check_approxidate 3.months.ago '2009-05-30 19:20:00' failure
-check_approxidate 2.years.3.months.ago '2007-05-30 19:20:00' failure
+check_approxidate 3.months.ago '2009-05-30 19:20:00'
+check_approxidate 2.years.3.months.ago '2007-05-30 19:20:00'
 
 check_approxidate '6am yesterday' '2009-08-29 06:00:00'
 check_approxidate '6pm yesterday' '2009-08-29 18:00:00'