fixup! fixup! Add basic test for date fields

cbits/field.c

@@ -123,9 +123,7 @@ static inline bool is_separator(char c) {
 }
 
 /* Match a year in the 20xx century, in big-endian date format with or
-   without separators.
-
-   precondition: n >= 8 */
+   without separators. */
 static inline bool match_ymd(const char *buf, size_t n) {
 	/* The shortest thing we're willing to call a "date" at this
 	   point is YYYYMMDD. */