Convert the mode more portably

The previous conversion worked on Linux but failed on macOS where
modes, as represented in system data structures, are 16-bit rather
than 32-bit.

The previous commit broke builds on macOS (and possibly other
OSes not currently tested on CI), which this should fix.

This commit also does some other refactorings:

- Simplify the way conversions are represented.

- Express with `expect` that, by this point, there are no unknown
  bits, rather than doing something that would silently preserve or
  silently discard them. There would have to be a bug (and probably
  in `gix-worktree-state` itself) for that not to be the case.

- Clarify the TODO comment, and also weaken it since there might be
  some reason for `set_mode_executable` to keep using `Permissions`
  in some way.

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

@@ -287,10 +287,11 @@ pub(crate) fn finalize_entry(
     if let Some(path) = set_executable_after_creation {
         let old_perm = std::fs::symlink_metadata(path)?.permissions();
         if let Some(new_perm) = set_mode_executable(old_perm) {
-            // TODO: If the `fchmod` approach is kept, `set_mode_executable` shouldn't operate on std::fs::Permissions.
-            use std::os::{fd::AsFd, unix::fs::PermissionsExt};
-            rustix::fs::fchmod(file.as_fd(), new_perm.mode().into())
-                .map_err(|errno| std::io::Error::from_raw_os_error(errno.raw_os_error()))?;
+            // 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())
+                .expect("`set_mode_executable` shouldn't preserve or add unknown bits");
+            rustix::fs::fchmod(&file, mode).map_err(std::io::Error::from)?;
         }
     }
     // NOTE: we don't call `file.sync_all()` here knowing that some filesystems don't handle this well.