Some changes fix actual problems, e.g. missing spaces in square bracket tests,
and backticks unintentionally causing code execution when intended as formatting.
Others, such as conservative quoting probably work fine in the old version in
most situations, but it's nice to have some additional safety.
Using `{ ..; }` instead of `(..)` is just a small performance enhancement.
Many of these issues were identified by shellcheck, which might be useful in CI
as well.
To allow paths with spaces, give a shell-quoted path to hyperfine. We
could make this a bit shorter by quoting as early as possible, like
FISH_PATH=$(quote "$1")
and then use $FISH_PATH unquoted. I discarded that idea because it
probably looks surprising.
Closes#8559
