fix: quote $split of $split && return for custom IFS

[akinomyoga]

These are all the following simple changes but just applied to many files.

-    $split && return
+    "$split" && return

This was originally a part of #914, but I have separated it for easier review of non-trivial changes in #914.

[akinomyoga]

Do we also switch to [[ $split ]] for split? This involves an interface change to _comp_initialize, but we have renamed it from _init_completion so we may write an adapter in 000_bash_completion_compat.sh.

[akinomyoga]

From #931 (comment) by @scop
Now that we're doing the boolean checks differently throughout for other cases, the _comp_initialize $split case looks inconsistent. I wonder if it would make sense and be worth the trouble to introduce another variable, e.g. $was_split or something with the empty/set value space we use for the other boolean checks? Would be a big diff though. Perhaps there would be an opportunity to do something about this as we've renamed _init_completion to _comp_initialize. What do you think?

[akinomyoga]

• master...akinomyoga:quote-split-B ... This is the version for the suggested interface change so that the variable split is set to an empty string or set by _comp_initialize.
• master...akinomyoga:quote-split-C ... This is the version with the further name change split => was_split.

[scop]

Ooh ❤️ thanks heaps! I think I'd prefer option C -- it reads slightly better to me, and could cause less confusion for 3rd party completion writers, in case they take some of our existing completions as a template but opt to use _init_completion for backwards compatibility. Which option would you prefer?

Let's just add a @deprecated note and elaboration to the new _init_completion wrapper.

[akinomyoga]

I pushed the version for option C (was_split).

Let's just add a @deprecated note and elaboration to the new _init_completion wrapper.

I added the original description and @deprecated with the way to migrate.

[scop]

Thanks!