rust-filepatterns: add comment about Windows path handling

As I replied to the Phabricator message, this is wrong. And I even suspect
it wouldn't compile because of multiple type mismatches.

I think, in Rust where type system is rock solid, we can live with UTF-8
strings except for the bottom storage layer and the top UI/command layer.
We'll still have to get around undecodable characters not to be lost, but
I think it's okay to drop such filenames from match result if they don't
match in UTF-8 world, not in Latin-1 world.

rust/hg-core/src/utils/files.rs

@@ -9,6 +9,8 b' pub fn get_path_from_bytes(bytes: &[u8])'
     }
     #[cfg(windows)]
     {
+        // TODO: convert from Windows MBCS (ANSI encoding) to WTF8.
+        // Perhaps, the return type would have to be Result<PathBuf>.
         use std::os::windows::ffi::OsStrExt;
         os_str = std::ffi::OsString::from_wide(bytes);
     }