For legacy bash completions, similarly as commit
4f0facbcee is for bash completions v2.
As a side effect, fixes test suite with shellcheck 0.8.0 installed;
apparently the 0.7.0 that's in GitHub Actions' ubuntu-latest at the
moment does not flag the array quoting related issue that was provoked
from 0.8.0 before this change.
Refactor to remove two loops over the entire list of candidates.
Format descriptions only for completions that are actually going to be
displayed, instead of for all candidates.
Format descriptions inline in completions array, removing need for a
command substitution/subshell and a printf escape per displayed
completion.
Ville Skyttä