Do a backup when forcing new file

bin/mkexec.pl

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 if (exists $bangs->{$type}) {
-    -e $name and not $force
-        or mkexec $name, mkbody($type);
+    $force and -e $name
+        and (defined `cp "$name" "$name~"` or die $!);
+    (not -e $name or $force)
+        and mkexec $name, mkbody($type);
     chmod $DEFAULT_MODE, $name;
     launch_editor $name;
 } else {