rust: fix file folding map

The file folding map, frequently used on macOS, had two issues:

* the means for converting it to Python didn't work
* a minor typo when copying the python code, where `!=` became `==`

With this, the rust code passes all tests on macOS.

Test Plan:
I'm currently doing a full test run on a case-insensitive file system. If it passes, perhaps we can change the platform check from an error to a warning?

Differential Revision: https://phab.mercurial-scm.org/D9671

rust/hg-core/src/dirstate/dirstate_map.rs

@@ -431,7 +431,7 b' impl DirstateMap {'
         let mut new_file_fold_map = FileFoldMap::default();
 
         for (filename, DirstateEntry { state, .. }) in self.state_map.iter() {
-            if *state == EntryState::Removed {
+            if *state != EntryState::Removed {
                 new_file_fold_map
                     .insert(normalize_case(&filename), filename.to_owned());
             }
@@ -447,7 +447,7 b' impl DirstateMap {'
         let mut new_file_fold_map = FileFoldMap::default();
 
         for (filename, DirstateEntry { state, .. }) in self.state_map.iter() {
-            if state == EntryState::Removed {
+            if state != EntryState::Removed {
                 new_file_fold_map
                     .insert(normalize_case(&filename), filename.to_owned());
             }

rust/hg-cpython/src/dirstate/dirstate_map.rs

@@ -350,8 +350,8 b' py_class!(pub class DirstateMap |py| {'
         {
             dict.set_item(
                 py,
-                key.as_bytes().to_vec(),
-                value.as_bytes().to_vec(),
+                PyBytes::new(py, key.as_bytes()).into_object(),
+                PyBytes::new(py, value.as_bytes()).into_object(),
             )?;
         }
         Ok(dict)