Fix --sort=ext

The logic of the previous version wasn't correct. Also, presuming
natural ordering of full filenames is still reasonable when the
extensions are identical.

src/options.rs

@@ -5,7 +5,6 @@ use column::Column::*;
 use output::{Grid, Details};
 use term::dimensions;
 
-use std::ascii::AsciiExt;
 use std::cmp::Ordering;
 use std::fmt;
 
@@ -118,13 +117,9 @@ impl FileFilter {
             SortField::Name => files.sort_by(|a, b| natord::compare(&*a.name, &*b.name)),
             SortField::Size => files.sort_by(|a, b| a.stat.size.cmp(&b.stat.size)),
             SortField::FileInode => files.sort_by(|a, b| a.stat.unstable.inode.cmp(&b.stat.unstable.inode)),
-            SortField::Extension => files.sort_by(|a, b| {
-                if a.ext.cmp(&b.ext) == Ordering::Equal {
-                    Ordering::Equal
-                }
-                else {
-                    a.name.to_ascii_lowercase().cmp(&b.name.to_ascii_lowercase())
-                }
+            SortField::Extension => files.sort_by(|a, b| match a.ext.cmp(&b.ext) {
+                Ordering::Equal => natord::compare(&*a.name, &*b.name),
+                order => order
             }),
             SortField::ModifiedDate => files.sort_by(|a, b| a.stat.modified.cmp(&b.stat.modified)),
             SortField::AccessedDate => files.sort_by(|a, b| a.stat.accessed.cmp(&b.stat.accessed)),