to_char(): have format 'OF' only show the leading negative sign

Previously both hours and minutes displayed as negative.

Report by David Pozsar

src/backend/utils/adt/formatting.c

@@ -2507,7 +2507,7 @@ DCH_to_char(FormatNode *node, bool is_interval, TmToChar *in, char *out, Oid col
 				s += strlen(s);
 				if (tm->tm_gmtoff % SECS_PER_HOUR != 0)
 				{
-					sprintf(s, ":%02ld", (tm->tm_gmtoff % SECS_PER_HOUR) / SECS_PER_MINUTE);
+					sprintf(s, ":%02ld", abs(tm->tm_gmtoff % SECS_PER_HOUR) / SECS_PER_MINUTE);
 					s += strlen(s);
 				}
 				break;