git: add `--without-completions` option

Some users prefer the `_git` script included with the zsh distribution
instead of the one based on the bash completion script.

References #16992.

Closes #18180.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
git: use `build.with?`.
upgrade hub to v1.10.5

Closes #18181.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
cleanup: don't remove old incomplete files twice.
Use https for GitHub URLs.
qt: fix superenv and other cleanup.
A more helpful error message for `brew untap`

`brew untap` without arguments currently errors out (as it should), but the
error message is generic and unhelpful. (The error message is borrowed from
a method in the tap command - `tap_args`.)

Closes #18186.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>