Document choice of script exit code

tests/scripts/curves.pl

@@ -36,6 +36,7 @@ my @curves = split( /\s+/, `sed -n -e '$sed_cmd' $config_h` );
 system( "cp $config_h $config_h.bak" ) and die;
 sub abort {
     system( "mv $config_h.bak $config_h" ) and warn "$config_h not restored\n";
+    # use an exit code between 1 and 124 for git bisect (die returns 255)
     warn $_[0];
     exit 1;
 }