Fix EXTRACT(ISOYEAR FROM timestamp) for years BC.

The test cases added by commit 26ae3aa exposed an old oversight in
timestamp[tz]_part: they didn't correct the result of date2isoyear()
for BC years, so that we produced an off-by-one answer for such years.
Fix that, and back-patch to all supported branches.

Discussion: https://postgr.es/m/SG2PR06MB37762CAE45DB0F6CA7001EA9B6550@SG2PR06MB3776.apcprd06.prod.outlook.com

Author: tglsfdc

src/backend/utils/adt/timestamp.c

@@ -4373,6 +4373,7 @@ date2isoweek(int year, int mon, int mday)
 /* date2isoyear()
  *
  *	Returns ISO 8601 year number.
+ *	Note: zero or negative results follow the year-zero-exists convention.
  */
 int
 date2isoyear(int year, int mon, int mday)
@@ -4647,6 +4648,9 @@ timestamp_part(PG_FUNCTION_ARGS)
 
 			case DTK_ISOYEAR:
 				result = date2isoyear(tm->tm_year, tm->tm_mon, tm->tm_mday);
+				/* Adjust BC years */
+				if (result <= 0)
+					result -= 1;
 				break;
 
 			case DTK_DOW:
@@ -4843,6 +4847,9 @@ timestamptz_part(PG_FUNCTION_ARGS)
 
 			case DTK_ISOYEAR:
 				result = date2isoyear(tm->tm_year, tm->tm_mon, tm->tm_mday);
+				/* Adjust BC years */
+				if (result <= 0)
+					result -= 1;
 				break;
 
 			case DTK_DOW:

src/test/regress/expected/timestamp.out

@@ -809,7 +809,7 @@ SELECT d1 as "timestamp",
  Fri Feb 14 17:32:01 1997    |      1997 |    7 |      5 |   5 |  45
  Sat Feb 15 17:32:01 1997    |      1997 |    7 |      6 |   6 |  46
  Sun Feb 16 17:32:01 1997    |      1997 |    7 |      7 |   0 |  47
- Tue Feb 16 17:32:01 0097 BC |       -96 |    7 |      2 |   2 |  47
+ Tue Feb 16 17:32:01 0097 BC |       -97 |    7 |      2 |   2 |  47
  Sat Feb 16 17:32:01 0097    |        97 |    7 |      6 |   6 |  47
  Thu Feb 16 17:32:01 0597    |       597 |    7 |      4 |   4 |  47
  Tue Feb 16 17:32:01 1097    |      1097 |    7 |      2 |   2 |  47

src/test/regress/expected/timestamptz.out

@@ -918,7 +918,7 @@ SELECT d1 as timestamptz,
  Fri Feb 14 17:32:01 1997 PST    |      1997 |    7 |      5 |   5 |  45
  Sat Feb 15 17:32:01 1997 PST    |      1997 |    7 |      6 |   6 |  46
  Sun Feb 16 17:32:01 1997 PST    |      1997 |    7 |      7 |   0 |  47
- Tue Feb 16 17:32:01 0097 PST BC |       -96 |    7 |      2 |   2 |  47
+ Tue Feb 16 17:32:01 0097 PST BC |       -97 |    7 |      2 |   2 |  47
  Sat Feb 16 17:32:01 0097 PST    |        97 |    7 |      6 |   6 |  47
  Thu Feb 16 17:32:01 0597 PST    |       597 |    7 |      4 |   4 |  47
  Tue Feb 16 17:32:01 1097 PST    |      1097 |    7 |      2 |   2 |  47