Improve fish completion script generation.

There are many issues with script generation. Some of them are documented in #679 & its comments.

This issue is for a first batch of fish completion script fixes.

I will submit a PR. Each commit should note the fix(es) that it contains.

After the PR for this issue is merged, I will sequentially create issues & submit PRs for cross-shell changes, then I will migrate the 3 shells to ToolInfo (unless @rauhul wants to do it), then I will resume working on more fixes (though I'm happy to pause if any of you want to modify the completion code or anything that interacts with it).