Fix side-by-side line length when colour is disabled

Fixes #406

Looks like this was inadvertently broken in #301.

CHANGELOG.md

@@ -24,6 +24,11 @@ displayed.
 `--list-languages` now respects the value of `--color`, whose default
 only uses styling when stdout is a TTY.
 
+### Display
+
+Fixed side-by-side display width when color is disabled (when piping
+difftastic stdout or with `--color=never`).
+
 ### Build
 
 Fixed an issue with building on Windows with gcc.

src/display/style.rs

@@ -106,15 +106,20 @@ pub fn split_and_apply(
     assert!(max_len > 0);
 
     if styles.is_empty() && !line.trim().is_empty() {
-        // Missing styles is a bug, so highlight in purple to make this obvious.
         return split_string_by_width(line, max_len, matches!(side, Side::Left))
             .into_iter()
-            .map(|(part, _)| {
+            .map(|(part, pad)| {
+                let mut res = String::with_capacity(part.len() + pad);
                 if use_color {
-                    highlight_missing_style_bug(part)
+                    // If we're syntax highlighting and have no
+                    // styles, that's a bug.
+                    res.push_str(&highlight_missing_style_bug(part));
                 } else {
-                    part.to_owned()
+                    res.push_str(part);
                 }
+
+                res.push_str(&" ".repeat(pad));
+                res
             })
             .collect();
     }