change: Set +x via open file descriptor, not path

When executable permissions were set on a file being checked out,
and when it was done after the file was already created, this was
done on its path, using `chmod` (via a higher level abstraction).
But we have an open `File` object for it already, at that point.

This uses `fchmod` on the existing open file descriptor (obtained
from the `File` object), instead of `chmod` on the path.

(Since #1764, the file mode has also been read first, to figure out
what new mode to set it to for +x. That uses `lstat` (via a higher
level abstraction). The change here is therefore really only half
of what should be done. To complete it, and also to avoid new
problems due to a new inconsistency, that should be done with
`fstat` on the file descriptor instead of `lstat` on the path. That
will be done in a directly fortcoming commit -- after portability
issues in the type of the `st_mode` field, which is not actually
`u32` on all systems, are examined.)

Author: EliahKagan

Cargo.lock

@@ -29,3 +29,9 @@ gix-filter = { version = "^0.17.0", path = "../gix-filter" }
 io-close = "0.3.7"
 thiserror = "2.0.0"
 bstr = { version = "1.3.0", default-features = false }
+
+[target.'cfg(unix)'.dependencies]
+rustix = { version = "0.38.20", default-features = false, features = [
+    "std",
+    "fs",
+] }

gix-worktree-state/Cargo.toml

@@ -287,7 +287,10 @@ 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) {
-            std::fs::set_permissions(path, new_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()))?;
         }
     }
     // NOTE: we don't call `file.sync_all()` here knowing that some filesystems don't handle this well.