rechecked by somebody who knows what is meant to happen here. The new libreadline rluser.texinfo file duplicates the '@defcodeindex bt' entry.
requires the new file. Fixed stale near-copy of contrib/libreadline/doc/hsuser.texinfo. Patch it at build ntime, and only keep the patch for it here. Don't keep a copy of contrib/gdb/gdb/doc/all-cfg.texi here. Link to it at build time.