Really convert the mode more portably

While replacing `into()` with `rustix::fs::Mode::from_bits(...)`
helped in expressing (and enforcing) the desired semantics for bits
that fit in `mode_t` but whose meaning is unrecognized, it did not
actually make the code build on macOS, and probably was not enough
to improve portability to any other systems where `mode_t` is not
`u32`.

This commit adds an explicit checked conversion of the value we get
from calling `mode()` on a `Permissions` object, which is aways
`u32`, into whatever type `mode_t` is defined as, which is `u32` on
most systems but `u16` on macOS and possibly others.

gix-worktree-state/src/checkout/entry.rs

@@ -289,7 +289,8 @@ pub(crate) fn finalize_entry(
         if let Some(new_perm) = set_mode_executable(old_perm) {
             // TODO: If we keep `fchmod`, maybe change `set_mode_executable` not to use `std::fs::Permissions`.
             use std::os::unix::fs::PermissionsExt;
-            let mode = rustix::fs::Mode::from_bits(new_perm.mode())
+            let raw_mode = new_perm.mode().try_into().expect("mode fits in `st_mode`");
+            let mode = rustix::fs::Mode::from_bits(raw_mode)
                 .expect("`set_mode_executable` shouldn't preserve or add unknown bits");
             rustix::fs::fchmod(&file, mode).map_err(std::io::Error::from)?;
         }