uniq: Fix skip fields

Current implementation of the skip fields logic does not handle
multibyte code points correctly. It assumes each code point (`char`) is
one byte. If the skipped part of the input line has any multibyte code
points then this can cause fields not being skipped correctly (field
start index is calculated to be before it actually starts).

src/uu/uniq/src/uniq.rs

@@ -79,22 +79,19 @@ impl Uniq {
 
     fn skip_fields<'a>(&self, line: &'a str) -> &'a str {
         if let Some(skip_fields) = self.skip_fields {
-            if line.split_whitespace().count() > skip_fields {
-                let mut field = 0;
-                let mut i = 0;
-                while field < skip_fields && i < line.len() {
-                    while i < line.len() && line.chars().nth(i).unwrap().is_whitespace() {
-                        i += 1;
-                    }
-                    while i < line.len() && !line.chars().nth(i).unwrap().is_whitespace() {
-                        i += 1;
-                    }
-                    field += 1;
+            let mut i = 0;
+            let mut char_indices = line.char_indices();
+            for _ in 0..skip_fields {
+                if char_indices.find(|(_, c)| !c.is_whitespace()) == None {
+                    return "";
+                }
+                match char_indices.find(|(_, c)| c.is_whitespace()) {
+                    None => return "",
+
+                    Some((next_field_i, _)) => i = next_field_i,
                 }
-                &line[i..]
-            } else {
-                ""
             }
+            &line[i..]
         } else {
             line
         }

tests/fixtures/uniq/skip-2-fields.expected

@@ -1,2 +1,2 @@
-	  aaa  aa  a
+	  aaa  ⟪⟫  a
 aa  a

tests/fixtures/uniq/skip-fields.txt

@@ -1,4 +1,4 @@
-	  aaa  aa  a
+	  aaa  ⟪⟫  a
 	  ZZZ     aa  a
 ZZZ     aa  a
 	  ZZZ     bb  a