Commit 69053d962 (Use menu behavior when completing change-directory,
2022-07-19) made ":cd dir/" actually run ":cd dir/first-subdir",
which can be surprising.
This is usually irrelevant because you rarely type the trailing slash.
However it does happen after correcting an error with `<backspace>`
and friends. For for example,
:cd d<tab>/f<backspace>
results in
:cd dir/
We should probably fix user expectations here. Do this by adding "dir/"
as valid completion. This requires us to allow empty candidates in
"RankedMatch" but there's no harm in that. This means we need to
filter out empty completions from shell-script-candidates elsewhere.
Alternative fix: we could revert 69053d962. This would remove the
convenient menu behavior but that wouldn't be a huge deal.
Fixes#4775
