Autocompletion on Bash on Mac not working

Each time I'm doing a TAB, I see things like this:

```
-bash: _get_comp_words_by_ref: command not found
```

I can avoid it by catching the errors:

```bash
$ cat ~/.bash.d/cht.sh
_cht_complete()
{
local cur prev opts
_get_comp_words_by_ref -n : cur 2>/dev/null

COMPREPLY=()
cur="${COMP_WORDS[COMP_CWORD]}"
prev="${COMP_WORDS[COMP_CWORD-1]}"
opts="$(curl -s cheat.sh/:list)"

if [ ${COMP_CWORD} = 1 ]; then
COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
__ltrim_colon_completions "$cur" 2>/dev/null
fi
return 0
}
complete -F _cht_complete cht.sh
```

Notice the `2>/dev/null` for both commands.

Start with the Bash completion entry point in ~/.bash.d/cht.sh and reproduce the TAB completion error on macOS. Check the availability of _get_comp_words_by_ref and __ltrim_colon_completions in the completion environment. Done means cht.sh completion works without requiring error suppression.

Written by the indexing model from the issue text.