parse_timestamp(): specify explicitly where we parse timestamps

Currently, Git's source code represents all timestamps as `unsigned
long`. In preparation for using a more appropriate data type, let's
introduce a symbol `parse_timestamp` (currently being defined to
`strtoul`) where appropriate, so that we can later easily switch to,
say, use `strtoull()` instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tag.c

@@ -110,8 +110,8 @@ static unsigned long parse_tag_date(const char *buf, const char *tail)
 		/* nada */;
 	if (buf >= tail)
 		return 0;
-	/* dateptr < buf && buf[-1] == '\n', so strtoul will stop at buf-1 */
-	return strtoul(dateptr, NULL, 10);
+	/* dateptr < buf && buf[-1] == '\n', so parsing will stop at buf-1 */
+	return parse_timestamp(dateptr, NULL, 10);
 }
 
 int parse_tag_buffer(struct tag *item, const void *data, unsigned long size)