in sv_gets() if the sv is empty then preallocate a small buffer

When the COW logic is triggered there is an unfortunate slowdown.

https://rt.perl.org/Ticket/Display.html?id=121796

What happens is that the buffer gets "stolen" by COW,
and $_ ends up with SvLEN()==0, which through a tortorious
pathway leads us to allocate the same amount of memory as there
is in the current buffer. Which then gets COW'ed out, and etc.

By initializing the string to at least something this does not happen.

sv_gets() seriously needs love. :-(

Author: demerphq

sv.c

@@ -46,6 +46,8 @@
   char *gconvert(double, int, int,  char *);
 #endif
 
+#define PERL_PREALLOC_EMPTY_IN_SV_GETS 20
+
 #ifdef PERL_NEW_COPY_ON_WRITE
 #   ifndef SV_COW_THRESHOLD
 #    define SV_COW_THRESHOLD	0	/* min string length for cow */
@@ -8154,6 +8156,10 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
     (*fp)->_cnt++;
 #endif
 
+    if (!SvLEN(sv)) {
+        SvGROW(sv,PERL_PREALLOC_EMPTY_IN_SV_GETS);
+    }
+
     /* Here is some breathtakingly efficient cheating */
 
     cnt = PerlIO_get_cnt(fp);			/* get count into register */