Short strings can be external, too

That complicates a little object equality (and therefore table access
for long strings), but the old behavior was somewhat weird. (Short
strings, a concept otherwise absent from the manual, could not be
external.)

lobject.h

@@ -418,6 +418,7 @@ typedef struct TString {
 
 
 #define strisshr(ts)	((ts)->shrlen >= 0)
+#define isextstr(ts)	(ttislngstring(ts) && tsvalue(ts)->shrlen != LSTRREG)
 
 
 /*