Improve the size of completions page to show the entire multi-line prompt

[s417-lama]

Description

Problem

When editing multi lines and selecting completions, the page of completions hides the lines except the bottom line.
I think it is uncomfortable not to be able to see the entire lines even after selecting the completion.

What I changed

I changed to show the entire lines when selecting completions.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[ridiculousfish]

Thanks, nice fix! I'm going to merge it as is, but then I will change it to use a simple newline count instead of the heavyweight prompt layout machinery.

Merged as 27c1c06 - thanks!